qemu-img: wait for convert coroutines to complete

On error path (like i/o error in one of the coroutines), it's required to
  - wait for coroutines completion before cleaning the common structures
  - reenter dependent coroutines so they ever finish

Introduced in 2d9187bc.

Cc: qemu-stable@nongnu.org
Signed-off-by: Anton Nefedov <anton.nefedov@virtuozzo.com>
Reviewed-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit b91127ed)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>