net/virtio-user: fix crash in server mode

net/virtio-user: fix crash in server mode

For virtio-user server mode, one use case comes across segmentation fault.
step 1: Launch vhost side as client firstly.
step 2: launch virtio-user side as server.

The cause is: after registering virtio_interrupt_handler into
eal-intr-thread, two threads (main thread and eal-intr-thread) have
sync issues, so add rxvq pointer checking in function virtio_notify_peers
to decide if the code can continue.

Fixes: bd8f50a45d0f ("net/virtio-user: support server mode")
Cc: stable@dpdk.org
Signed-off-by: Zhiyong Yang <zhiyong.yang@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>