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

OvmfPkg/OvmfPkg.dec

@@ -44,6 +44,7 @@
   gEfiXenInfoGuid                 = {0xd3b46f3b, 0xd441, 0x1244, {0x9a, 0x12, 0x0, 0x12, 0x27, 0x3f, 0xc1, 0x4d}}
 
 [Protocols]
+  gVirtioDeviceProtocolGuid       = {0xfa920010, 0x6785, 0x4941, {0xb6, 0xec, 0x49, 0x8c, 0x57, 0x9f, 0x16, 0x0a}}
   gBlockMmioProtocolGuid          = {0x6b558ce3, 0x69e5, 0x4c67, {0xa6, 0x34, 0xf7, 0xfe, 0x72, 0xad, 0xbe, 0x84}}
 
 [PcdsFixedAtBuild]