Add code to make handling domain poweroff/reboot symmetrical between

Add code to make handling domain poweroff/reboot symmetrical between
paravirtualized and fully virtualized.  This approach uses the new
sched_op to handle other domains than the current domain.  The new
code, SCHEDOP_remote_shutdown, is very much like SCHEDOP_shutdown, but
is called with the id of the domain which is to be shut down. This
allows fully virtualized shutdown and para-virtualized shutdown to be
identical from that point forward.

A paravirtualized domain uses sched_op to shut down and set the reason
code. This will send a VIRQ_DOM_EXC, which can be handled in dom0 by
control software.  In some ways, this resembles SIGCHILD/waitpid, and
is a reasonable model.

The fully virtualized case has qemu invoke xm directly. This is a
different path than paravirtualized. It also removes decision and
policy making choices from the rest of the control software and places
it within qemu. When any dom0 logic eventually gets a VIRQ_DOM_EXC,
the information about the domain is gone having been destroyed by xm.

A libxenctrl wrapper, xc_shutdown_domain has been added and qemu now
calls it.

As a freebie, #if 0 some very verbose logging code in qemu.  Totally
unrelated, but as long as I was there...

Signed-off-by: Ben Thomas <ben@virtualiron.com>