MAINTAINERS: adjust x86/mm/shadow maintainers

Better reflect reality: Andrew and Jan are active maintainers
and I review patches.  Keep myself as a reviewer so I can help
with historical context &c.

Signed-off-by: Tim Deegan <tim@xen.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

MAINTAINERS

@@ -591,7 +591,9 @@ F:	xen/arch/x86/mm/mem_sharing.c
 F:	tools/tests/mem-sharing/
 
 X86 SHADOW PAGETABLES
-M:	Tim Deegan <tim@xen.org>
+M:	Jan Beulich <jbeulich@suse.com>
+M:	Andrew Cooper <andrew.cooper3@citrix.com>
+R:	Tim Deegan <tim@xen.org>
 S:	Maintained
 F:	xen/arch/x86/mm/shadow/