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>