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>