OvmfPkg/Virtio.h: Added the macros that define the Device Specific Configuration...

OvmfPkg/Virtio.h: Added the macros that define the Device Specific Configuration Offset

The Device Specific Configuration region is located at different locations
for the VirtIo devices over PCI, PCI with MSI-X capability, MMIO.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14807 6f19259b-4bc3-4df7-8a09-765794883524