vm: avoid duplicate import

Vm._getVmPolicy added another minidom import just to use parseString.
There is no need for it since we already imported it as _domParseStr
from a long time.

Change-Id: I000b19bb46cd140372492d552f27a25424dd97e8
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31380
Reviewed-by: Dan Kenigsberg <danken@redhat.com>