Both Boris and David have graciously volunteered to help in
maintaining the Xen subsystem tree. Cementing this in the
MAINTAINERS file so they are copied on Xen related patches.

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