PowerPC : Add support for launching VM in 'compat' mode.

PowerPC : Add support for launching VM in 'compat' mode.

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: Prerna Saxena <prerna@linux.vnet.ibm.com>
Signed-off-by: Li Zhang <zhlcindy@linux.vnet.ibm.com>
Signed-off-by: Pradipta Kr. Banerjee <bpradip@in.ibm.com>
Acked-by: Michal Privoznik <mprivozn@redhat.com>