Change listen address on libvirt live-migration

Previously, in order for libvirt live migration to work,
the graphics listen addresses (VNC and/or Spice) had to be
set on both machines to be 0.0.0.0.  This was because libvirt
would try to rebind to the source address on the destination
machine, but would fail due to the source machine already having
that address (this would not happen for 0.0.0.0, however).

Now, a modified copy of the instance's XML is passed to libvirt's
migrate function, such that the listen addresses are that of the
destination machine.  The addresses are stashed in the pre-migration
data by the destination pre-migration method.

Note that this functionality will only work on versions of libvirt
with the VIR_DOMAIN_XML_MIGRATABLE flag.  If the flag is not detected
(anything vanilla libvirt less than 1.0.0), the live migration code
will fall back to the old method.

Fixes: bug #1279563

Change-Id: I6c9500283027193179c11d18ad4242e165a9ad6e

Conflicts:
	nova/tests/virt/libvirt/test_libvirt.py
	nova/virt/libvirt/driver.py