OvmfPkg/Virtio10Dxe: add the RingBaseShift offset

OvmfPkg/Virtio10Dxe: add the RingBaseShift offset

virtio drivers use VIRTIO_DEVICE_PROTOCOL.MapSharedBuffer() to map the
ring buffer host address to a device address. If an IOMMU is present then
RingBaseShift contains the offset from the host address.

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>