Fix virsh migrate tests

Hi, this patch series fixes the migration tests.

Migrations needs a shared storage disk and with this new option 'shared_storage_nfs' you can specify path to this disk file which should be the same on both hosts.

Also we should skip direct migrate test for hypervisor different from xen.

And the last modification is that we let libvirt to generate mac address for the attached network interface to test the --xml option.