xen: add save/restore/migrate for Xen domains

Hi Ingo,

This series adds save/restore/migrate for Xen domains.  This is almost
entirely contained within the Xen-specific code.

The only exception is the WARN_ON I remove in hres_timers_resume,
which is spurious in the case of a Xen suspend/resume, since we don't
offline all the CPUs.

This also adds another user of the freezer, since it turns out to
solve a somewhat tricky problem.  The problem is that Xen pagetables
are expressed in terms of machine frame numbers - ie, the page numbers
of the underlying host machine.  When the domain is saved, all the
mfns everywhere are canonicalized into guest-relative pfns in the save
image.  The tricky part is that when preemption is enabled, it's
possible the suspend will preempt a pagetable manipulation, which will
leave a machine frame number sitting in a register or on the stack
which will not be canonicalized.

The freezer solves this because it makes sure that every thread is
quiesced at a well known point, which is not during a pagetable
update.  So by freezing everything and then stopping all the
processors in stop_machine, we can make sure the system is in a
saveable state.

The freezer is only required when preemption is enabled, since
stop_machine will quiesce all processors (and therefore threads) at
suitable points anyway.

     J