MAINTAINERS: Add myself to the Xen Hypervisor Interface and remove Chris Wright.

Chris is working on other stuff now, and I am working full-time
with Jeremy on these bits.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: Chris Wright <chrisw@sous-sol.org>