solve virtio_inject_irq's concurrency issue

[liulog]

Related to #68.

void virtio_inject_irq(VirtQueue *vq) {
    ......
    pthread_mutex_lock(&RES_MUTEX);
    while (is_queue_full(virtio_bridge->res_front,
                         virtio_bridge->res_rear,
                         MAX_REQ)) {
    }
    ......
    pthread_mutex_unlock(&RES_MUTEX);
    ......
}

When multiple threads arrive simultaneously, the thread that acquires the lock is given priority.

It continuously polls is_queue_full rather than having multiple threads fairly contend for the lock continuously.

This logic is sound here.

Given that access to the critical section is exclusive to this location, it is acceptable for the lock to be held for long durations.

[KouweiLee]

close #68