block: Clean up after deleting BHs

block: Clean up after deleting BHs

Commit 6a7ad299 ("Call qemu_bh_delete at bdrv_aio_bh_cb") deletes emulated
aio bottom halves to prevent endless accumulation.  However, it leaves a
stale ->bh pointer, which is then waited on when the aio is reused.

Zeroing the pointer fixes the issue, allowing vmdk format images to be used.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
[ Backported from 6bbff9a0b495918309074ac60375be5f9dc868b3
  by Stefano Stabellini. ]

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
(cherry picked from commit a05958b6e32f1748ea70b1efca13394956c0698b)