virsh_migrate: add option to specify path to nfs disk file

To migrate a guest you need to have a shared disk file between both
hosts. To accomplish this, you need to use a nfs server.
virsh_migrate: remove static mac address for "xml" test

We are using static mac address "ff:ff:ff:ff:ff:ff" which is broadcast
address, but libvirt neads unicast address. Instead of using static mac
address we can leave libvirt to generate one and then obtain that mac
address for further checking in the code.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
virsh_migrate: skip direct migration for driver different from xen

Direct migration is supported only for xen in libvirt and therefore
we should skip this test for all other drivers instead of failing.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>