fix iommu was added automatically when cpus are bigger than 255

[nanli1]

LIBVIRT-1414 says case update Signed-off-by: nanli [email protected]

Before fixed:

avocado run --vt-type libvirt --vt-omit-data-loss --vt-machine-type q35 max_vcpus.negative_test.with_iommu --vt-connect-uri qemu:///system
 (1/1) type_specific.io-github-autotest-libvirt.max_vcpus.negative_test.with_iommu: FAIL: Expect should fail with one of IOMMU interrupt remapping requires split I/O APIC \(ioapic driver='qemu'\), but succeeded: Domain 'avocado-vt-vm1' defined from /var/tmp/xml_utils_temp_91mfmfls.xml\n\n\n (6.13 s)

After fixed: (1/1) type_specific.io-github-autotest-libvirt.max_vcpus.negative_test.with_iommu: PASS (5.87 s)