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.