Maintainers.txt: split out section "OvmfPkg: Xen-related modules"

Maintainers.txt: split out section "OvmfPkg: Xen-related modules"

We ask contributors to copy Anthony and Julien on patches that affect "Xen
modules" under OvmfPkg. With the "GetMaintainer.py" script being
introduced later, express this expectation in "Maintainers.txt" in
machine-readable format.

Cc: Andrew Fish <afish@apple.com>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Julien Grall <julien.grall@arm.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>