xen: update missing ioctl magic numers documentation

Add missing ioctl "magic numbers" for various Xen interfaces
(xenbus_dev.h, gntalloc.h, gntdev.h, and privcmd.h).

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: xen-devel@lists.xenproject.org
Link: https://lore.kernel.org/r/20220131161959.16509-1-rdunlap@infradead.org


Signed-off-by: Juergen Gross <jgross@suse.com>