ArmVirtPkg/ArmVirtXen: move from Intel to generic BDS

ArmVirtPkg/ArmVirtXen: move from Intel to generic BDS

ArmVirtXen is the only remaining consumer of ArmPlatformPkg's
PlatformIntelBdsLib implementation, which is tightly coupled to the
deprecated Intel BDS. So move ArmVirtXen to the generic BDS as well,
allowing us to get rid of PlatformIntelBdsLib entirely.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Julien Grall <julien.grall@linaro.org>