OvmfPkg/VirtioDevice.h: Introduced VIRTIO_DEVICE_PROTOCOL protocol

OvmfPkg/VirtioDevice.h: Introduced VIRTIO_DEVICE_PROTOCOL protocol

This protocol introduces an abstraction to access the VirtIo
Configuration and Device spaces.
The registers in these spaces are located at a different offset and have
a different width whether the transport layer is either PCI or MMIO. This
protocol would also allow to support VirtIo PCI devices with MSI-X
capability in a transparent way (Device space is at a different offset
when a PCIe device has MSI-X capability).

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
v5:
- add disclaimer (two instances) about the protocol being work in progress

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@14963 6f19259b-4bc3-4df7-8a09-765794883524