tests: initial tests for vm libvirt callbacks

This patch adds initial tests for the libvirt callbacks
of vdsm.virt.Vm.

The dispatching logic for the libvirt callbacks in clientIF
is not covered by this patch.

Change-Id: I059ef6cd42e8cfa03e2891dade00ff21e4c435bb
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27128
Reviewed-by: Dan Kenigsberg <danken@redhat.com>