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".