OvmfPkg/Virtio10: define VIRTIO_F_IOMMU_PLATFORM feature bit

OvmfPkg/Virtio10: define VIRTIO_F_IOMMU_PLATFORM feature bit

This feature indicates that the device is behind an IOMMU that translates
bus addresses from the device into physical addresses in memory.  If this
feature bit is set to 0, then the device emits physical addresses which
are not translated further, even though an IOMMU may be present.
see [1] for more infromation

[1] https://lists.oasis-open.org/archives/virtio-dev/201610/msg00121.html

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>