Fix CPU compare check bug in live migration

The current implemention for livirt live migration CPU check compares
both source and destination node physical CPU which is too strict,
the correct way is to compare the CPU of instance to be migrated
with CPU of the destination node.

This patch adds a get_instance_cpu_config method to virt driver which
returns None by default, libvirt driver implement this to return guest
cpu model info in xml format.

Closes-Bug: 1082414

Change-Id: I65f505fec64c65d2641d2bfd940cde44bcd83a78

Conflicts:
	nova/compute/manager.py
	nova/compute/rpcapi.py
	nova/virt/driver.py