Automate discard_granularity disk attribute cases

[chunfuwen]

Automate discard_granularity disk attribute cases scenarios cover normal, negative, boundary and updates xx-300179 - Live update disk with //disk/blockio[@discard_granularity xx-300180 - Define&&Start vm with //disk/blockio[@discard_granularity xx-300181 - Hotplug/unplug disk with //disk/blockio[@discard_granularity xx-300202 - Define&&Start vm with //disk/blockio[@discard_granularity]- invalid values xx-300203 - Define&&Start vm with //disk/blockio[@discard_granularity] - boundary values

[chunfuwen]

PASS 01-type_specific.io-github-autotest-libvirt.virtual_disks.discard_granularity.coldplug.file.normal_vm_tart.virtio PASS 02-type_specific.io-github-autotest-libvirt.virtual_disks.discard_granularity.coldplug.file.normal_vm_tart.sata PASS 03-type_specific.io-github-autotest-libvirt.virtual_disks.discard_granularity.coldplug.file.normal_vm_tart.scsi PASS 05-type_specific.io-github-autotest-libvirt.virtual_disks.discard_granularity.coldplug.file.failure_vm_tart.virtio PASS 06-type_specific.io-github-autotest-libvirt.virtual_disks.discard_granularity.coldplug.file.failure_vm_tart.sata PASS 07-type_specific.io-github-autotest-libvirt.virtual_disks.discard_granularity.coldplug.file.failure_vm_tart.scsi PASS 08-type_specific.io-github-autotest-libvirt.virtual_disks.discard_granularity.coldplug.file.failure_vm_tart.usb PASS 09-type_specific.io-github-autotest-libvirt.virtual_disks.discard_granularity.coldplug.file.boundary_vm_tart.value_zero.virtio PASS 10-type_specific.io-github-autotest-libvirt.virtual_disks.discard_granularity.coldplug.file.boundary_vm_tart.value_minus.virtio PASS 11-type_specific.io-github-autotest-libvirt.virtual_disks.discard_granularity.coldplug.file.boundary_vm_tart.value_max.virtio PASS 12-type_specific.io-github-autotest-libvirt.virtual_disks.discard_granularity.coldplug.file.boundary_vm_tart.value_max_minus_one.virtio PASS 13-type_specific.io-github-autotest-libvirt.virtual_disks.discard_granularity.coldplug.file.live_update_discard_granularity.add_discard_granularity.virtio PASS 14-type_specific.io-github-autotest-libvirt.virtual_disks.discard_granularity.coldplug.file.live_update_discard_granularity.modify_discard_granularity.virtio PASS 15-type_specific.io-github-autotest-libvirt.virtual_disks.discard_granularity.coldplug.file.live_update_discard_granularity.remove_discard_granularity.virtio PASS 16-type_specific.io-github-autotest-libvirt.virtual_disks.discard_granularity.coldplug.vhostuser.failure_vm_tart.virtio PASS 17-type_specific.io-github-autotest-libvirt.virtual_disks.discard_granularity.hotplug.file.normal_vm_tart.virtio PASS 18-type_specific.io-github-autotest-libvirt.virtual_disks.discard_granularity.hotplug.file.normal_vm_tart.scsi

[dzhengfy]

@cliping Could you help review?