xen: don't bother trying to set cr4

Xen ignores all updates to cr4, and some versions will kill the domain if
you try to change its value.  Just ignore all changes.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>