MAINTAINERS: Make me the Xen block subsystem (front and back) maintainer

The other MAINTAINER entries don't cover for this so the
xen-blkback driver looks like it is unmaintained. That is
not the case - but to make it obvious to tools - here is
the proper entry.

CC: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

MAINTAINERS

@@ -10032,6 +10032,13 @@ S:	Supported
 F:	arch/x86/pci/*xen*
 F:	drivers/pci/*xen*
 
+XEN BLOCK SUBSYSTEM
+M:	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+L:	xen-devel@lists.xenproject.org (moderated for non-subscribers)
+S:	Supported
+F:	drivers/block/xen-blkback/*
+F:	drivers/block/xen*
+
 XEN SWIOTLB SUBSYSTEM
 M:	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
 L:	xen-devel@lists.xenproject.org (moderated for non-subscribers)