MAINTAINERS: Update the Xen subsystem's with proper mailing list.

And also drop the virtualization one since we don't really use it.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com> [for netback]
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>