thread-pool: add missing qemu_bh_cancel in completion function

commit 3c80ca15 fixed a deadlock scenarion with nested aio_poll invocations.

However, the rescheduling of the completion BH introcuded unnecessary spinning
in the main-loop. On very fast file backends this can even lead to the
"WARNING: I/O thread spun for 1000 iterations" message popping up.

Callgrind reports about 3-4% less instructions with this patch running
qemu-img bench on a ramdisk based VMDK file.

Fixes: 3c80ca15
Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit b7a745dc)
* drop context dep on b9e413dd
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>