vhost: move the device ready check at proper place

vhost: move the device ready check at proper place

Currently, we check vq->desc, vq->kickfd and vq->callfd to know whether
a virtio device is ready or not. However, we only do it when handling
SET_VRING_KICK message, which could be wrong if a vhost-user frontend
send SET_VRING_KICK first and SET_VRING_CALL later.

To work for all possible vhost-user frontend implementations, we could
move the ready check at the end of vhost-user message handler.

Meanwhile, since we do the check more often than before, the "virtio
not ready" message is dropped, to not flood the screen.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>