Fix Xen panic with oprofile

Fix Xen panic with oprofile

The attached patch fixes a Xen panic when a domain is shutdown before
oprofile is stopped. Without this patch, free_xenoprof_pages() is
called before the domain is destroyed and that, in turn, prevents
oprofile from cleaning up pages shared with guests. Shutting down a
domain without terminating oprofile therefore causes a Xen panic at a
later point in time.

Signed-off-by: Niraj Tolia <niraj.tolia@hp.com>