xen-blkback,xen-blkfront: add myself as maintainer

I've done quite a lot of work in blkfront/blkback, and I usually end up
looking at the patches, so add myself as maintainer together with Konrad.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>