MAINTAINERS: update XEN HYPERVISOR INTERFACE

I am no longer in a postion to be a maintainer of the Xen subsystem.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Juergen Gross <jgross@suse.com>
Acked-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>