virtio: optimize Tx enqueue
virtio: optimize Tx enqueue

All the error checks in virtqueue_enqueue_xmit are already done
by the caller. Therefore they can be removed to improve performance.

Signed-off-by: Stephen Hemminger <06fa905d7f2aaced6dc72e9511c71a2a51e8aead@networkplumber.org>
Acked-by: Yuanhan Liu <5dcef21195e64c9c08aa779bb62b25e9133eaa33@linux.intel.com>
Acked-by: Huawei Xie <3b3be4428106761300a2ee559c9f71ecdb091553@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk