ArmVirtPkg/PlatformCI: factor out reusable PlatformBuildLib.py

ArmVirtPkg/PlatformCI: factor out reusable PlatformBuildLib.py

In order to reduce the amount of code duplication, refactor the
PlatformBuild.py script that builds ArmVirtQemu.dsc into a reusable
PlatformBuildLib.py containing most of the bits and pieces, and a small
QemuBuild.py which is specific to the DSC in question.

Suggested-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Michael Kubacki <michael.kubacki@microsoft.com>