ArmVirtPkg/ArmVirtXen: don't set Pcd*ImageVerificationPolicy

ArmVirtPkg/ArmVirtXen: don't set Pcd*ImageVerificationPolicy

According to the

  PCDs not used by modules or in conditional directives

sections of all the build reports for

  {AARCH64,ARM} x {Xen} x {DEBUG,NOOPT,RELEASE} x {feat-1}

(6 builds in total), PcdOptionRomImageVerificationPolicy,
PcdFixedMediaImageVerificationPolicy, and
PcdRemovableMediaImageVerificationPolicy are not used in any of those
builds.

Restrict the settings to the ArmVirtQemu and ArmVirtQemuKernel platforms
(preserving the -D SECURE_BOOT_ENABLE restriction in the process).

("feat-1" stands for "-D HTTP_BOOT_ENABLE -D NETWORK_IP6_ENABLE -D
SECURE_BOOT_ENABLE -D TTY_TERMINAL", while "feat-0" stands for "".)

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