OvmfPkg/VirtioGpuDxe: don't unmap VRING & BackingStore at ExitBootServices

OvmfPkg/VirtioGpuDxe: don't unmap VRING & BackingStore at ExitBootServices

In one of the following patches, we'll change OvmfPkg/IoMmuDxe so that it
unmaps all existent bus master operations (CommonBuffer, Read, Write) at
ExitBootServices(), strictly after the individual device drivers abort
pending DMA on the devices they manage, in their own ExitBootServices()
notification functions.

In preparation, remove the explicit
VIRTIO_DEVICE_PROTOCOL.UnmapSharedBuffer() calls from VirtioGpuExitBoot(),
originally added in commit 9bc5026c19a5 ("OvmfPkg/VirtioGpuDxe: map VRING
for bus master common buffer operation", 2017-08-26) and commit
f10ae923665f ("OvmfPkg/VirtioGpuDxe: map backing store to bus master
device address", 2017-08-26).

Add a DEBUG message so we can observe the ordering between
VirtioGpuExitBoot() and the upcoming cleanup of mappings in
OvmfPkg/IoMmuDxe.

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