janitorial: move isVdsmImage into utils

this patch moves vdsm/virt/vm.isVdsmImage into utils without code
changes. In doing so, we bootstrap the vdsm/virt/utils module,
which will contain the common shared code in the virt package,
to shed some load from vm.py.

This change is a preliminary step to the move of VmSamplingThread
from vm.py to sampling.py.

Change-Id: I9cb288eef41b567da36849e00f848e1ba20a62af
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28477
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>