Maintainers.txt: add Xen reviewers to OvmfPkg

Maintainers.txt: add Xen reviewers to OvmfPkg

A lack of reviewers with Xen experience for OvmfPkg has been expressed.
The Xen community has put forward two volunteers.

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Julien Grall <julien.grall@arm.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>