drive mirror:fix memory leak

In the function mirror_iteration() -> qemu_iovec_init(),
it allocates memory for op->qiov.iov, when the write request calls back,
but in the function mirror_iteration_done(), it only frees the op,
not free the op->qiov.iov, so this causes memory leak.

It should use qemu_iovec_destroy() to free op->qiov.

Signed-off-by: Zhang Min <rudy.zhangmin@huawei.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
