OvmfPkg/VirtioGpuDxe: helpers for backing store (de)allocation+(un)mapping
Introduce the VirtioGpuAllocateZeroAndMapBackingStore() and
VirtioGpuUnmapAndFreeBackingStore() helper functions. These functions tie
together the allocation, zeroing and mapping, and unmapping and
deallocation, respectively, of memory that the virtio GPU will permanently
reference after receiving the RESOURCE_ATTACH_BACKING command.
With these functions we can keep the next patch simpler -- the GOP
implementation in "Gop.c" retains its error handling structure, and
remains oblivious to VIRTIO_DEVICE_PROTOCOL and VirtioLib.