PowerISA allows processors to run VMs in binary compatibility ("compat")
mode supporting an older version of ISA. QEMU has recently added support to
explicitly denote a VM running in compatibility mode through commit 6d9412ea
& 8dfa3a5e85. Now, a "compat" mode VM can be run by invoking this qemu
commandline on a POWER8 host:  -cpu host,compat=power7.

This patch allows libvirt to exploit cpu mode 'host-model' to describe this
new mode for PowerKVM guests. For example, when a user wants to request a
power7 vm to run in compatibility mode on a Power8 host, this can be
described in XML as follows :

  <cpu mode='host-model'>
    <model>power7</model>
  </cpu>
Signed-off-by: NPrerna Saxena <prerna@linux.vnet.ibm.com>
Signed-off-by: NLi Zhang <zhlcindy@linux.vnet.ibm.com>
Signed-off-by: NPradipta Kr. Banerjee <bpradip@in.ibm.com>
Acked-by: NMichal Privoznik <mprivozn@redhat.com>