MAINTAINERS: xen: Konrad to step down and Juergen to pick up

I've lately been concentrating on other projects and haven't been
doing much of Xen core maintainership for the last year.

I am quite thrilled that Juergen is willing to help out!

P.S.
I am still the maintainer of Xen-SWIOTLB, Xen PCI-[front|backend],
and co-maintainer of Xen block-[front|backend]; amongst others.

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