vm: add boot menu support

libvirt supports interactive boot menu at guest startup,
depending on the support of the underlying hypervisor
and/or BIOS.
The boot menu can be used to select the boot device
priority.

This patch exposes the boot menu setting at VDSM level.

Change-Id: I4a845097294603bbb11e7941995cb1729f388109
Bug-Url: https://bugzilla.redhat.com/1047624
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23558
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Martin Betak <mbetak@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>