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>
