monitor: check return value of qemu_find_net_clients_except()

qemu_find_net_clients_except() may return a value which is greater
than the size of array we provided. So we should check this value
before using it, otherwise this may cause unexpected memory access.

This patch fixes the net related command completion when we have a
virtio-net nic with more than 255 queues.

Cc: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
