OvmfPkg: VirtioNetDxe: adapt virtio-net packet header size to virtio-1.0

In virtio-0.9.5, the size of the virtio-net packet header depends on
whether the VIRTIO_NET_F_MRG_RXBUF feature is negotiated -- the
"num_buffers" field is only appended to the header if the feature is
negotiated.

Since we never negotiate this feature, VirtioNetDxe never allocates room
for the "num_buffers" field.

With virtio-1.0, the "num_buffers" field is always there (although it
doesn't carry useful information without VIRTIO_NET_F_MRG_RXBUF). Adapt
the buffers that depend on the virtio-net header size (otherwise we have
skewed / truncated packets).

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

OvmfPkg/VirtioNetDxe/VirtioNet.h

@@ -89,7 +89,7 @@ typedef struct {
   UINT16                      TxMaxPending;      // VirtioNetInitTx
   UINT16                      TxCurPending;      // VirtioNetInitTx
   UINT16                      *TxFreeStack;      // VirtioNetInitTx
-  VIRTIO_NET_REQ              TxSharedReq;       // VirtioNetInitTx
+  VIRTIO_1_0_NET_REQ          TxSharedReq;       // VirtioNetInitTx
   UINT16                      TxLastUsed;        // VirtioNetInitTx
 } VNET_DEV;