virtio: fix crash if VIRTIO_NET_F_CTRL_VQ is not negotiated
virtio: fix crash if VIRTIO_NET_F_CTRL_VQ is not negotiated

If VIRTIO_NET_F_CTRL_VQ is not negotiated hw->cvq will be NULL

Signed-off-by: Damjan Marion <9141bba8b2efed526e55cf796d48631af330ad98@cisco.com>
Acked-by: Changchun Ouyang <938eba6db5c8b5dd06c4584560dc75bc6c54c681@intel.com>
Acked-by: Olivier Matz <dfcc1510895413197abb4336e39bb0a3906cba71@6wind.com>

mixja/dpdk,msune/dpdk,john-mcnamara-intel/dpdk,venkynv/dpdk-mirror,venkynv/dpdk-mirror,tsphillips/dpdk-fork,tsphillips/dpdk-fork,john-mcnamara-intel/dpdk,msune/dpdk,msune/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,tsphillips/dpdk-fork,mixja/dpdk,venkynv/dpdk-mirror,msune/dpdk,venkynv/dpdk-mirror,mixja/dpdk,mixja/dpdk,tsphillips/dpdk-fork