MAINTAINERS: Update Xen-[PCI,SWIOTLB,Block] maintainership

Konrad's new job role is putting a serious cramp on him
being a responsive maintainer and as such he is handing off
the reins to Juergen, Roger, and Stefano.

Thank you!

Acked-by: Juergen Gross <jgross@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>