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: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index d8b0f36..e3d4513 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -273,9 +273,12 @@
 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
 
 Hosts:
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