vvfat: make it thread-safe

write_target_commit(BlockDriverState *bs, uint64_t offset, uint64_t bytes,

                    QEMUIOVector *qiov, int flags)

{

    int ret;

    BDRVVVFATState* s = *((BDRVVVFATState**) bs->opaque);

    return try_commit(s);

    qemu_co_mutex_lock(&s->lock);

    ret = try_commit(s);

    qemu_co_mutex_unlock(&s->lock);

    return ret;

}

static void write_target_close(BlockDriverState *bs) {