qed: Simplify request handling

Now that we process a request in the same coroutine from beginning to
end and don't drop out of it any more, we can look like a proper
coroutine-based driver and simply call qed_aio_next_io() and get a
return value from it instead of spawning an additional coroutine that
reenters the parent when it's done.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

block/qed.h

@@ -129,8 +129,7 @@ enum {
 };
 
 typedef struct QEDAIOCB {
-    BlockAIOCB common;
-    int bh_ret;                     /* final return status for completion bh */
+    BlockDriverState *bs;
     QSIMPLEQ_ENTRY(QEDAIOCB) next;  /* next request */
     int flags;                      /* QED_AIOCB_* bits ORed together */
     uint64_t end_pos;               /* request end on block device, in bytes */