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
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31353
Reviewed-by: Dan Kenigsberg <danken@redhat.com>