Maintainers.txt: split out section "ArmVirtPkg: modules used on Xen"

Maintainers.txt: split out section "ArmVirtPkg: modules used on Xen"

We ask contributors to copy Julien on patches that affect "Xen modules"
under ArmVirtPkg. 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: Ard Biesheuvel <ard.biesheuvel@linaro.org>
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>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>