MAINTAINERS: update Xen files

Add the PV block backend, the Xen mapcache, and hw/i386/xen to the list
of Xen related files maintained by me.
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>

MAINTAINERS

@@ -273,9 +273,12 @@ F: */xen*
 F: hw/char/xen_console.c
 F: hw/display/xenfb.c
 F: hw/net/xen_nic.c
+F: hw/block/xen_*
 F: hw/xen/
 F: hw/xenpv/
+F: hw/i386/xen/
 F: include/hw/xen/
+F: include/sysemu/xen-mapcache.h
 
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