OvmfPkg/VirtioRngDxe: don't unmap VRING 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() call from VirtioRngExitBoot(),
    originally added in commit 0a568ccbcbd1 ("OvmfPkg/VirtioRngDxe: map host
    address to device address", 2017-08-23).
    
    Add a DEBUG message so we can observe the ordering between
    VirtioRngExitBoot() 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>