block migration: do not submit multiple AIOs for same sector

block migration: do not submit multiple AIOs for same sector

Block migration can submit multiple AIO reads for the same sector/chunk, but
completion of such reads can happen out of order:

migration               guest
- get_dirty(N)
- aio_read(N)
- clear_dirty(N)
                        write(N)
                        set_dirty(N)
- get_dirty(N)
- aio_read(N)

If the first aio_read completes after the second, stale data will be
migrated to the destination.

Fix by not allowing multiple AIOs inflight for the same sector.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>