MAINTAINERS: add xen config fragments to XEN HYPERVISOR sections

Make changes to the xen config fragments reach the XEN HYPERVISOR
maintainers and mailing list.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20220810050712.9539-5-lukas.bulwahn@gmail.com


Signed-off-by: Juergen Gross <jgross@suse.com>