virtio: restrict vector Rx/Tx to x86 SSSE3

virtio: restrict vector Rx/Tx to x86 SSSE3

Temporary implementation to let virtio operate in non-vec mode for archs
which doesn't support _ssse_ cpuflag.

todo:
1) Move virtio_recv_pkts_vec() implementation to
   drivers/virtio/virtio_vec_<arch>.h file.
2) Remove use_simple_rxtx flag, so that virtio/virtio_vec_<arch>.h
   files to provide vectored/non-vectored rx/tx apis.

Fixes: fc3d66212fed ("virtio: add vector Rx")
Fixes: c121c8d6d31a ("virtio: add simple Tx")
Fixes: 8d8393fb1861 ("virtio: pick simple Rx/Tx")
Signed-off-by: Santosh Shukla <sshukla@mvista.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>