OvmfPkg/VirtioMmioDeviceLib: implement IOMMU-like member functions
    
    The patch implements the newly added IOMMU-like member functions by
    respectively delegating the job to:
    
    - VIRTIO_DEVICE_PROTOCOL.AllocateSharedPages () ->
        MemoryAllocationLib.AllocatePages()
    
    - VIRTIO_DEVICE_PROTOCOL.FreeSharedPages () ->
        MemoryAllocationLib.FreePages ()
    
    - VIRTIO_DEVICE_PROTOCOL.MapSharedBuffer () -> no-op
    
    - VIRTIO_DEVICE_PROTOCOL.UnmapSharedBuffer () -> no-op
    
    Suggested-by: Laszlo Ersek <lersek@redhat.com>
    Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Tom Lendacky <thomas.lendacky@amd.com>
    Cc: Laszlo Ersek <lersek@redhat.com>
    Contributed-under: TianoCore Contribution Agreement 1.1
    Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
    Reviewed-by: Laszlo Ersek <lersek@redhat.com>
    Regression-tested-by: Laszlo Ersek <lersek@redhat.com>