OvmfPkg/VirtioMmioDeviceLib: implement IOMMU-like member functions

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>