libvirt: Fix live block migration
libvirt: Fix live block migration

Current check_can_live_migrate_destination trys to get instance info
by call self._conn.lookupByName(instance_name) on the destination
host before migrating. Because the instance is not on the destination
host yet, it failes with InstanceNotFound.

This fix gets the available disk size of the destination side.
On the source side, checks whether the above space is enough before
migrating.

Fixes bug 1044237

Change-Id: I20f64e1f85828049b697a4b1173bac8e5779d45a

n0ano/ganttclient