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 <7437296077edea1cb64451c122c4dac071f0cfbc@redhat.com>
Cc: Ard Biesheuvel <66d3c5fdaeea7ff1f996ad04f2c45e08ab38e2f5@linaro.org>
Cc: Jordan Justen <a79006884ba9cf5f4f7fcbb92a15b208780d8c97@intel.com>
Cc: Tom Lendacky <bcbe2d8614c98cf419bfc4320d4a7a260d53de8f@amd.com>
Cc: Laszlo Ersek <7437296077edea1cb64451c122c4dac071f0cfbc@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Brijesh Singh <6c1ba716601dd19ee463759bfdaf6de1e69b2b21@amd.com>
Reviewed-by: Laszlo Ersek <7437296077edea1cb64451c122c4dac071f0cfbc@redhat.com>
Regression-tested-by: Laszlo Ersek <7437296077edea1cb64451c122c4dac071f0cfbc@redhat.com>

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