rbd: Fix race between aio completition and aio cancel

This one fixes a race which qemu had also in iscsi block driver
between cancellation and io completition.

qemu_rbd_aio_cancel was not synchronously waiting for the end of
the command.

To archieve this it introduces a new status flag which uses
-EINPROGRESS.

Signed-off-by: Stefan Priebe <s.priebe@profihost.ag>
Reviewed-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
