xen/x86: support XENPF_settime64

Try XENPF_settime64 first, if it is not available fall back to
XENPF_settime32.

No need to call __current_kernel_time() when all the info needed are
already passed via the struct timekeeper * argument.

Return NOTIFY_BAD in case of errors.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>