MAINTAINERS: Add V section entry to allow identification of Xen file

MAINTAINERS: Add V section entry to allow identification of Xen file

This change provides sufficient information to allow get_maintainer.pl /
add_maintainers.pl scripts to be run on xen sister repositories such as
mini-os.git, osstest.git, etc

A suggested template for sister repositories of Xen is

========================================================
This file follows the same conventions as outlined in
xen.git:MAINTAINERS. Please refer to the file in xen.git
for more information.

THE REST
M:      MAINTAINER1 <maintainer1@email.com>
M:      MAINTAINER2 <maintainer2@email.com>
L:      xen-devel@lists.xenproject.org
S:      Supported
F:      *
F:      */
V:      xen-maintainers-1
========================================================

Signed-off-by: Lars Kurth <lars.kurth@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>