kvm: do not add disk cache mode from an empty string

[DaanHoogland]

Description

This PR fixes a bug discussed in https://github.com/apache/cloudstack/discussions/12226#discussioncomment-15220839

Types of changes

• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] Enhancement (improves an existing feature and functionality)
• [ ] Cleanup (Code refactoring and cleanup, that may add test cases)
• [ ] Build/CI
• [ ] Test (unit or integration test code)

Feature/Enhancement Scale or Bug Severity

Feature/Enhancement Scale

• [ ] Major
• [ ] Minor

Bug Severity

• [ ] BLOCKER
• [ ] Critical
• [ ] Major
• [ ] Minor
• [ ] Trivial

Screenshots (if appropriate):

How Has This Been Tested?

How did you try to break this feature and the system with this change?

[codecov[bot]]

Codecov Report

:x: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 16.21%. Comparing base (79ebf69) to head (6aada70).

Files with missing lines	Patch %	Lines
...ypervisor/kvm/resource/LibvirtDomainXMLParser.java	0.00%	2 Missing :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##               4.20   #12248      +/-   ##
============================================
- Coverage     16.21%   16.21%   -0.01%     
+ Complexity    13354    13351       -3     
============================================
  Files          5657     5657              
  Lines        498642   498643       +1     
  Branches      60520    60521       +1     
============================================
- Hits          80868    80862       -6     
- Misses       408754   408769      +15     
+ Partials       9020     9012       -8     

Flag	Coverage Δ
uitests	4.00% <ø> (ø)
unittests	17.07% <0.00%> (-0.01%)	:arrow_down:

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[weizhouapache]

@blueorangutan package

[blueorangutan]

@weizhouapache a [SL] Jenkins job has been kicked to build packages. It will be bundled with KVM, XenServer and VMware SystemVM templates. I'll keep you posted as I make progress.

[blueorangutan]

Packaging result [SF]: ✔️ el8 ✔️ el9 ✔️ el10 ✔️ debian ✔️ suse15. SL-JID 16017

[weizhouapache]

@blueorangutan test

[blueorangutan]

@weizhouapache a [SL] Trillian-Jenkins test job (ol8 mgmt + kvm-ol8) has been kicked to run smoke tests

[blueorangutan]

[SF] Trillian test result (tid-14976) Environment: kvm-ol8 (x2), zone: Advanced Networking with Mgmt server ol8 Total time taken: 52497 seconds Marvin logs: https://github.com/blueorangutan/acs-prs/releases/download/trillian/pr12248-t14976-kvm-ol8.zip Smoke tests completed. 132 look OK, 9 have errors, 0 did not run Only failed and skipped tests results shown below:

Test	Result	Time (s)	Test File
test_uservm_host_control_state	Failure	16.77	test_host_control_state.py
ContextSuite context=TestHostControlState>:teardown	Error	32.21	test_host_control_state.py
test_01_sys_vm_start	Failure	0.10	test_secondary_storage.py
ContextSuite context=TestCpuCapServiceOfferings>:setup	Error	0.00	test_service_offerings.py
ContextSuite context=TestServiceOfferings>:setup	Error	4.71	test_service_offerings.py
test_04_nonsecured_to_secured_vm_migration	Error	392.49	test_vm_life_cycle.py
test_08_migrate_vm	Error	46.90	test_vm_life_cycle.py
test_01_migrate_vm_strict_tags_success	Error	332.34	test_vm_strict_host_tags.py
test_01_create_redundant_VPC_2tiers_4VMs_4IPs_4PF_ACL	Error	104.10	test_vpc_redundant.py
test_02_redundant_VPC_default_routes	Error	102.02	test_vpc_redundant.py
test_03_create_redundant_VPC_1tier_2VMs_2IPs_2PF_ACL_reboot_routers	Error	112.62	test_vpc_redundant.py
test_04_rvpc_network_garbage_collector_nics	Error	113.27	test_vpc_redundant.py
test_05_rvpc_multi_tiers	Error	62.45	test_vpc_redundant.py
test_01_VPC_nics_after_destroy	Error	57.44	test_vpc_router_nics.py
test_02_VPC_default_routes	Error	58.15	test_vpc_router_nics.py
test_01_redundant_vpc_site2site_vpn	Failure	58.43	test_vpc_vpn.py
test_01_vpc_site2site_vpn_multiple_options	Failure	56.38	test_vpc_vpn.py
test_01_cancel_host_maintenance_ssh_enabled_agent_connected	Failure	1.14	test_host_maintenance.py
test_03_cancel_host_maintenance_ssh_disabled_agent_connected	Failure	0.14	test_host_maintenance.py
test_04_cancel_host_maintenance_ssh_disabled_agent_disconnected	Failure	1.13	test_host_maintenance.py