OvmfPkg/VirtioGpuDxe: map VRING for bus master common buffer operation

OvmfPkg/VirtioGpuDxe: map VRING for bus master common buffer operation

VirtioGpuDxe uses one virtio ring, for VIRTIO_GPU_CONTROL_QUEUE.

Map it for bus master common buffer operation with VirtioRingMap(), so
that it can be accessed equally by both guest and hypervisor even if an
IOMMU is used. (VirtioRingInit() already allocates the ring suitably for
this, see commit b0338c53297c, "OvmfPkg/VirtioLib: alloc VRING buffer with
AllocateSharedPages()", 2017-08-23).

Pass the resultant translation offset ("RingBaseShift"), from system
memory address to bus master device address, to VIRTIO_SET_QUEUE_ADDRESS.

Unmap the ring in all contexts where the ring becomes unused (these
contexts are mutually exclusive):

- in VirtioGpuInit(): the ring has been mapped, but we cannot complete the
  virtio initialization for another reason,

- in VirtioGpuUninit(): the virtio initialization has succeeded, but
  VirtioGpuDriverBindingStart() fails for another reason, or
  VirtioGpuDriverBindingStop() unbinds the device after use,

- in VirtioGpuExitBoot(): ExitBootServices() is called after
  VirtioGpuDriverBindingStart() has successfully bound the device.
  (Unmapping the ring does not change the UEFI memory map.)

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Brijesh Singh <brijesh.singh@amd.com>