Description of problem:
the phenom CPU model defined in QEMU contains "npt" feature while libvirt doesn't know anything about the feature. When a new domain is started, QEMU disables this feature on host, which is ignored by libvirt (it doesn't know the feature), but once restoring the domain it sees QEMU unexpectedly disabled some feature and complains about it.

Version-Release number of selected component (if applicable):
libvirt-3.2.0-14.el7_4.2.x86_64

How reproducible:
100% (on some machines)

Steps to Reproduce:
virsh # capabilities 
<capabilities>

  <host>
    <uuid>00020003-0004-0005-0006-000700080009</uuid>
    <cpu>
      <arch>x86_64</arch>
      <model>Opteron_G2</model>
      <vendor>AMD</vendor>
      <topology sockets='1' cores='2' threads='1'/>
      <feature name='vme'/>
      <feature name='ht'/>
      <feature name='mmxext'/>
      <feature name='fxsr_opt'/>
      <feature name='3dnowext'/>
      <feature name='3dnow'/>
      <feature name='cmp_legacy'/>
      <feature name='extapic'/>
      <feature name='cr8legacy'/>
      <pages unit='KiB' size='4'/>
      <pages unit='KiB' size='2048'/>
    </cpu>

1.Config domain xml as:
# virsh dumpxml avocado-vt-vm1
...
  <cpu mode='host-model' check='partial'>
    <model fallback='allow'/>
  </cpu>
...

2.Start guest, check xml:
# virsh dumpxml avocado-vt-vm1
...
  <cpu mode='custom' match='exact' check='full'>
    <model fallback='forbid'>phenom</model>
    <vendor>AMD</vendor>
    <feature policy='require' name='vme'/>
    <feature policy='require' name='cx16'/>
    <feature policy='require' name='x2apic'/>
    <feature policy='require' name='tsc-deadline'/>
    <feature policy='require' name='hypervisor'/>
    <feature policy='require' name='arat'/>
    <feature policy='require' name='lahf_lm'/>
    <feature policy='require' name='cmp_legacy'/>
    <feature policy='require' name='cr8legacy'/>
    <feature policy='disable' name='monitor'/>
    <feature policy='disable' name='svm'/>
    <feature policy='disable' name='popcnt'/>
    <feature policy='disable' name='pdpe1gb'/>
    <feature policy='disable' name='rdtscp'/>
    <feature policy='disable' name='abm'/>
    <feature policy='disable' name='sse4a'/>
  </cpu>
...

# virsh dumpxml avocado-vt-vm1 --migratable
...
  <cpu mode='custom' match='exact' check='partial'>
    <model fallback='forbid'>phenom</model>
    <vendor>AMD</vendor>
    <feature policy='require' name='vme'/>
    <feature policy='require' name='cx16'/>
    <feature policy='require' name='x2apic'/>
    <feature policy='require' name='tsc-deadline'/>
    <feature policy='require' name='hypervisor'/>
    <feature policy='require' name='arat'/>
    <feature policy='require' name='lahf_lm'/>
    <feature policy='require' name='cmp_legacy'/>
    <feature policy='require' name='cr8legacy'/>
    <feature policy='disable' name='monitor'/>
    <feature policy='disable' name='svm'/>
  </cpu>
...

3.Do managedsave:
# virsh managedsave avocado-vt-vm1

Domain avocado-vt-vm1 state saved by libvirt

4.Try to start the domain:
# virsh start avocado-vt-vm1 
error: Failed to start domain avocado-vt-vm1
error: operation failed: guest CPU doesn't match specification


Actual results:
As in step4, starting the domain from a managedsave state failed with error that cpu doesn't match

Expected results:
Should succeed to start the domain without error.

Additional info:
1.Also reproduces on the libvirt 3.2.0-14.
2.Not reproduce on libvirt-3.2.0-13.el7.x86_64