OvmfPkg/VirtioPciDeviceDxe: Implement VIRTIO_DEVICE_PROTOCOL for VirtIo Devices over PCI

OvmfPkg/VirtioPciDeviceDxe: Implement VIRTIO_DEVICE_PROTOCOL for VirtIo Devices over PCI

This change implements the VIRTIO_DEVICE_PROTOCOL for the PCI transport
layer.
The VirtIo device drivers will interact with the PCI-based VirtIo devices
through this protocol implementation.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
v5:
- updated comment block on VirtioPciDeviceRead()
- return EFI_UNSUPPORTED instead of failed ASSERT() in
  VirtioPciSetPageSize()
- VirtioPciIoRead(): restore the original requirement that FieldSize equal
  BufferSize exactly (not only divide it). The looping added in v4 did not
  match the comment block, and the only place that used it in v4 (ie.
  VirtioNetGetFeatures()) needs an open-coded loop anyway (will be done in
  a later part of v5).

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14964 6f19259b-4bc3-4df7-8a09-765794883524