xen: enable keyhandlers to work without register set specified

xen: enable keyhandlers to work without register set specified

There are only two keyhandlers which make use of the cpu_user_regs
struct passed to them. In order to be able to call any keyhandler in
non-interrupt contexts, too, modify those two handlers to cope with a
NULL regs pointer by using run_in_exception_handler() in that case.

Suggested-by: Julien Grall <julien@xen.org>
Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>