vm: fix getVmPolicy return type.

in case of error, getVmPolicy will return a VDSM API return
value, which may confuse the caller.
This patch makes getVmPolicy returns explicitely None on error,
and let the caller handle this condition and bail out correctly.

Change-Id: I4c1d79283ee2de4a5dcf54f3544ec346c68837e1
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29930
Reviewed-by: Martin Sivák <msivak@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>