Description of problem:
Cmd "virsh Hypervisor-cpu-compare" outputs wrong result with VM's active dumpxml as input because of topoext

Version-Release number of selected component (if applicable):
qemu-kvm-4.1.0-10.module+el8.1.0+4234+33aa4f57.x86_64
libvirt-5.6.0-5.virtcov.el8.x86_64
kernel-4.18.0-141.el8.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Check domcapabilities, there is no topoext cpu feature here

1. virsh domcapabilities
   ...
   <cpu>
   <mode name='host-passthrough' supported='yes'/>
   <mode name='host-model' supported='yes'>
   <model fallback='forbid'>EPYC-IBPB</model>
   <vendor>AMD</vendor>
   <feature policy='require' name='x2apic'/>
   <feature policy='require' name='tsc-deadline'/>
   <feature policy='require' name='hypervisor'/>
   <feature policy='require' name='tsc_adjust'/>
   <feature policy='require' name='arch-capabilities'/>
   <feature policy='require' name='cmp_legacy'/>
   <feature policy='require' name='perfctr_core'/>
   <feature policy='require' name='invtsc'/>
   <feature policy='require' name='virt-ssbd'/>
   <feature policy='require' name='skip-l1dfl-vmentry'/>
   <feature policy='disable' name='monitor'/>
   <feature policy='disable' name='svm'/>
   </mode>

2. Prepare a shutdown VM with the following conf

1. virsh domstate test_1
   shut off

1. virsh dumpxml test_1 --inactive |grep "<cpu" -A2
   <cpu mode='host-model' check='partial'>
   <model fallback='allow'/>
   </cpu>

4. Start VM and check active VM's dumpxml, topoext is enabled here

1. virsh start test_1
   Domain test_1 started

1. virsh dumpxml test_1 |grep "<cpu" -A20
   <cpu mode='custom' match='exact' check='full'>
   <model fallback='forbid'>EPYC-IBPB</model>
   <vendor>AMD</vendor>
   <feature policy='require' name='x2apic'/>
   <feature policy='require' name='tsc-deadline'/>
   <feature policy='require' name='hypervisor'/>
   <feature policy='require' name='tsc_adjust'/>
   <feature policy='require' name='arch-capabilities'/>
   <feature policy='require' name='cmp_legacy'/>
   <feature policy='require' name='perfctr_core'/>
   <feature policy='require' name='virt-ssbd'/>
   <feature policy='require' name='skip-l1dfl-vmentry'/>
   <feature policy='disable' name='monitor'/>
   <feature policy='disable' name='svm'/>
   <feature policy='require' name='topoext'/>
   </cpu>

5. Use the active VM's dumpxml as input of "virsh hypervisor-cpu-compare"

1. virsh dumpxml test_1 > test_1.xml

1. virsh hypervisor-cpu-compare test_1.xml
   CPU described in test_1.xml is incompatible with the CPU provided by hypervisor on the host

Actual results:
As step-5 shows

Expected results:
Since VM can start successfully on this host, I think the CPU conf should not be incompatible.

Additional info:
1> This issue can be reproduced on both RHEL-8.1 AV and RHEL-8.1.
2> The reproducing step above is for RHEL-8.1 AV, and the reproducing steps for RHEL-8.1 can be seen in this link: https://bugzilla.redhat.com/show_bug.cgi?id=1619798#c9