vm: do not use _dom for powerdown

We must avoid the proliferation of trivial helper methods, but
using private VM fields across modules is a greater evil, even
if modules are part of the same package and affine, like vm.py and
vmpowerdown.py

To avoid that, a new simple method is added to request clean
shutdown of a VM through ACPI events.

Change-Id: Ia99a8a557b9bbbaf693eb0e89aa9f39c0ecce400
Bug-Url: https://bugzilla.redhat.com/557125
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31901
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>