MAINTAINERS: add git repository for Xen

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

MAINTAINERS

@@ -9544,6 +9544,7 @@ M:	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
 M:	Boris Ostrovsky <boris.ostrovsky@oracle.com>
 M:	David Vrabel <david.vrabel@citrix.com>
 L:	xen-devel@lists.xenproject.org (moderated for non-subscribers)
+T:	git git://git.kernel.org/pub/scm/linux/kernel/git/xen/tip.git
 S:	Supported
 F:	arch/x86/xen/
 F:	drivers/*/xen-*front.c