HVM: Do not watch control/shutdown to force domain shutdowns when PV

HVM: Do not watch control/shutdown to force domain shutdowns when PV
drivers are not present. The feature-foo mechanism does not actually
work after save/restore since the fields do not get rewritten. And it
doesn't work when we actually need the guest to be shutdown through
Xen (e.g., on suspend). Since we can quite reasonably assume that the
only entity to screw with control/shutdown is xend, and since the one
place that does that now explicitly handles unenlightened HVM guests,
we can simply strip all this stuff out.

This has the nice benefit of making HVM save/restore with PV drivers
much more reliable.

Signed-off-by: Keir Fraser <keir@xensource.com>