MAINTAINERS: Update mailing list name for Xen pieces.

While the 'xen-devel@lists.xen.org' is more apt, it is not yet
ready. Revert the name back to the old lists.xensource.com for
right now.
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>

MAINTAINERS

@@ -6613,7 +6613,7 @@ F:	drivers/xen/*swiotlb*
 XEN HYPERVISOR INTERFACE
 M:	Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
 M:	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
-L:	xen-devel@lists.xen.org
+L:	xen-devel@lists.xensource.com
 L:	virtualization@lists.osdl.org
 S:	Supported
 F:	arch/x86/xen/