MAINTAINERS: add myself as a Xen maintainer

I have made many significant contributions to the Xen code in QEMU,
particularly the recent patches introducing a new PV device framework.
I intend to make further significant contributions, porting other PV back-
ends to the new framework with the intent of eventually removing the
legacy code. It therefore seems reasonable that I become a maintainer of
the Xen code.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Anthony Perard <anthony.perard@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>