OvmfPkg/IndustryStandard: add type definitions for the virtio GPU device

The GPU additions to VirtIo 1.0 are a work in progress. Mark the relevant
URLs in the source code. Incorporate the absolute minimum from the WIP
spec that is necessary for implementing a GOP driver.

Add the VIRTIO_SUBSYSTEM_GPU_DEVICE macro to
"IndustryStandard/Virtio10.h", since all other such macros (dating back to
VirtIo 0.9.5) are part of "IndustryStandard/Virtio095.h".

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Ref: https://tianocore.acgmultimedia.com/show_bug.cgi?id=66
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>