Description of problems:
Migrate a guest from rhel7.2 to rhel7.3 on host which support cpu feature 'arat', then try to migrate it back, met "error: internal error: Unknown CPU feature arat".


Version-Release number of selected component (if applicable):
libvirt-2.0.0-8.el7.x86_64
qemu-kvm-rhev-2.6.0-23.el7.x86_64

libvirt-1.2.17-13.el7_2.5.x86_64
qemu-kvm-rhev-2.3.0-31.el7_2.21.x86_64

How reproducible:
100%

1. Start a VM on rhel7.2 on host which support cpu feature 'arat'.
# virsh dumpxml r7|grep "<cpu" -A 5
  <cpu mode='host-model'>
    <model fallback='allow'/>
  </cpu>
# ps -ef|grep qemu
qemu      25019      1 93 01:53 ?        00:00:18 /usr/libexec/qemu-kvm -name r7... -cpu Haswell-noTSX,
+abm,+pdpe1gb,+rdrand,+f16c,+osxsave,+dca,+pdcm,+xtpr,+tm2,+est,+smx,+vmx,+ds_cpl,+monitor,+dtes64,+pbe,
+tm,+ht,+ss,+acpi,+ds,+vme -m 1024

2. Migrate the VM to rhel7.3
# virsh migrate r7 qemu+ssh://ipaddress/system --live --verbose
Migration: [100 %]

On the target host:
# virsh list --all
 Id    Name                           State
----------------------------------------------------
 1     r7                             running

# virsh dumpxml r7|grep "<cpu" -A 30
  <cpu mode='host-model'>
    <model fallback='allow'>Haswell-noTSX</model>
    <vendor>Intel</vendor>
    <feature policy='require' name='abm'/>
    <feature policy='require' name='pdpe1gb'/>
    <feature policy='require' name='rdrand'/>
    <feature policy='require' name='f16c'/>
    <feature policy='require' name='osxsave'/>
    <feature policy='require' name='dca'/>
    <feature policy='require' name='pdcm'/>
    <feature policy='require' name='xtpr'/>
    <feature policy='require' name='tm2'/>
    <feature policy='require' name='est'/>
    <feature policy='require' name='smx'/>
    <feature policy='require' name='vmx'/>
    <feature policy='require' name='ds_cpl'/>
    <feature policy='require' name='monitor'/>
    <feature policy='require' name='dtes64'/>
    <feature policy='require' name='pbe'/>
    <feature policy='require' name='tm'/>
    <feature policy='require' name='ht'/>
    <feature policy='require' name='ss'/>
    <feature policy='require' name='acpi'/>
    <feature policy='require' name='ds'/>
    <feature policy='require' name='vme'/>
  </cpu>
# ps -ef|grep qemu
qemu      35934      1  2 02:00 ?        00:00:03 /usr/libexec/qemu-kvm -name guest=r7...-cpu Haswell-noTSX,+vme,+ds,+acpi,+ss,+ht,+tm,+pbe,+dtes64,+monitor,
+ds_cpl,+vmx,+smx,+est,+tm2,+xtpr,+pdcm,+dca,+osxsave,+f16c,+rdrand,+pdpe1gb,
+abm -m 1024

3. Tried to migrate VM back to rhel7.2
# virsh migrate r7 qemu+ssh://10.73.131.71/system --live --verbose
error: internal error: Unknown CPU feature arat

Check the dumpxml --migratable, there are 'arat','tsc_adjust','xsaveopt' added.
# virsh dumpxml r7 --migratable
<domain type='kvm'>
  <name>r7</name>
.....
  <cpu mode='host-model' match='exact'>
    <model fallback='allow'>Haswell-noTSX</model>
    <vendor>Intel</vendor>
    <feature policy='require' name='vme'/>
    <feature policy='require' name='ds'/>
    <feature policy='require' name='acpi'/>
    <feature policy='require' name='ss'/>
    <feature policy='require' name='ht'/>
    <feature policy='require' name='tm'/>
    <feature policy='require' name='pbe'/>
    <feature policy='require' name='dtes64'/>
    <feature policy='require' name='monitor'/>
    <feature policy='require' name='ds_cpl'/>
    <feature policy='require' name='vmx'/>
    <feature policy='require' name='smx'/>
    <feature policy='require' name='est'/>
    <feature policy='require' name='tm2'/>
    <feature policy='require' name='xtpr'/>
    <feature policy='require' name='pdcm'/>
    <feature policy='require' name='dca'/>
    <feature policy='require' name='osxsave'/>
    <feature policy='require' name='f16c'/>
    <feature policy='require' name='rdrand'/>
    <feature policy='require' name='arat'/>
    <feature policy='require' name='tsc_adjust'/>
    <feature policy='require' name='xsaveopt'/>
    <feature policy='require' name='pdpe1gb'/>
    <feature policy='require' name='abm'/>
  </cpu>

Actual results:
In step3: Failed to migrate back to rhel7.2

Expected results:
In step3: Migrate back to rhel7.2 successfully.

Other information:
1) Test on host which support cpu features: arat, tsc_adjust,xsaveopt, they are new features in cpu_map.xml in rhel7.3
2) New features in rhel7.3 cpu_map.xml are:
tsc_adjust, cmt, clflushopt, xsaveopt, arat, xsavec,xgetbv1, xsaves, mbm_total, mbm_local.