qcow2: Remove request from in-flight list after error

If we complete a request with a failure we need to remove it from the list of
requests that are in flight. If we don't do it, the next time the same AIOCB is
used for a cluster allocation it will create a loop in the list and qemu will
hang in an endless loop.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
