blockdev: use drained_begin/end for qmp_block_resize

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1447551

If one tries to issue a block_resize while a guest is busy
accessing the disk, it is possible that qemu may deadlock
when invoking aio_poll from both the main loop and the iothread.

Replace another instance of bdrv_drain_all that doesn't
quite belong.

Cc: qemu-stable@nongnu.org
Suggested-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
(cherry picked from commit 698bdfa0)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

blockdev.c

@@ -2923,10 +2923,9 @@ void qmp_block_resize(bool has_device, const char *device,
         goto out;
     }
 
-    /* complete all in-flight operations before resizing the device */
-    bdrv_drain_all();
-
+    bdrv_drained_begin(bs);
     ret = blk_truncate(blk, size, errp);
+    bdrv_drained_end(bs);
 
 out:
     blk_unref(blk);