virtio: coverity woes -- divide by zero
virtio: coverity woes -- divide by zero

Coverity complains the expression, j % vif->num_txq, may encounter
divide by zero. While there is little chance that vif->num_txq is
zero, it is easy to prevent divide by zero if vif->num_txq is ever
zero.

Type: fix
Fixes: I337ec63d0868f665329d68eadf1744e080b73a0d

Signed-off-by: Steven Luong <a748ea44ad7b179c2313a856d44e1e4eb39869bb@cisco.com>
Change-Id: I2e91f296737ce266ab70fffc1f442cc600724fa2

chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp