MAINTAINERS: update Xen entries

moved in 56e2cd24..28b99f47 to hw/xen/ and hw/i386/xen/

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20170629150308.22766-4-f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>