Description of problem:
I try to make live snapshot by libvirt,the libvirt sometimes report an error :qemuMonitorJSONCheckError:377 : internal error: unable to execute QEMU command 'transaction': An IO error has occurred.

Version-Release number of selected component (if applicable):
qemu 1.7.1

How reproducible:
sometimes

Steps to Reproduce:
1.virsh start snapshot-test.vm
2.create snapshot.xml file:
<domainsnapshot>
  <description>test</description>
  <disks>
    <disk name='vda' snapshot="external">
      <source dev='/home/disk/sbd8' file='/home/disk/sdb8' type="block"/>
    </disk>
  </disks>
</domainsnapshot>
3.virsh snapshot-create snapshot-test.vm snapshot.xml --no-metadata --disk-only --reuse-external

Additional info:
I have read the qemu code about the snapshot create, and I find the qemu when call the function handle_aiocb_rw_linear():
static ssize_t handle_aiocb_rw_linear(RawPosixAIOData *aiocb, char *buf)
{
    ssize_t offset = 0;
    ssize_t len;

    while (offset < aiocb->aio_nbytes) {
        if (aiocb->aio_type & QEMU_AIO_WRITE) {
            len = pwrite(aiocb->aio_fildes,
                         (const char *)buf + offset,
                         aiocb->aio_nbytes - offset,
                         aiocb->aio_offset + offset);
        } else {
            len = pread(aiocb->aio_fildes,
                        buf + offset,
                        aiocb->aio_nbytes - offset,
                        aiocb->aio_offset + offset);
        }
        if (len == -1 && errno == EINTR) {
            continue;
        } else if (len == -1) {
            offset = -errno;
            break;
        } else if (len == 0) {
            break;
        }
        offset += len;
    }

    return offset;
}

The function pwrite happen error,the errono is 1,and the error describe:"pwrite failed, Operation not permitted (1, EPERM) because the process does not have the appropriate privileges to use the pwrite system call".
The qemu call stack about is:
external_snapshot_prepare()->bdrv_flush()->...->paio_submit->...->handle_aiocb_rw_linear.