ArmVirtPkg/ArmVirtQemu: refer to Shell app via its declared GUID

Currently, the file GUID reference of the UEFI Shell app is indirected
via the PCD gEfiIntelFrameworkModulePkgTokenSpaceGuid.PcdShellFile,
which is set to a fixed value for our platforms.

So instead, use the new symbolic GUID added for this purpose, and drop
the reference to this PCD, and to the IntelFrameworkModulePkg package
entirely.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>