block: Drain BH in bdrv_drained_begin

During block job completion, nothing is preventing
block_job_defer_to_main_loop_bh from being called in a nested
aio_poll(), which is a trouble, such as in this code path:

    qmp_block_commit
      commit_active_start
        bdrv_reopen
          bdrv_reopen_multiple
            bdrv_reopen_prepare
              bdrv_flush
                aio_poll
                  aio_bh_poll
                    aio_bh_call
                      block_job_defer_to_main_loop_bh
                        stream_complete
                          bdrv_reopen

block_job_defer_to_main_loop_bh is the last step of the stream job,
which should have been "paused" by the bdrv_drained_begin/end in
bdrv_reopen_multiple, but it is not done because it's in the form of a
main loop BH.

Similar to why block jobs should be paused between drained_begin and
drained_end, BHs they schedule must be excluded as well.  To achieve
this, this patch forces draining the BH in BDRV_POLL_WHILE.

As a side effect this fixes a hang in block_job_detach_aio_context
during system_reset when a block job is ready:

    #0  0x0000555555aa79f3 in bdrv_drain_recurse
    #1  0x0000555555aa825d in bdrv_drained_begin
    #2  0x0000555555aa8449 in bdrv_drain
    #3  0x0000555555a9c356 in blk_drain
    #4  0x0000555555aa3cfd in mirror_drain
    #5  0x0000555555a66e11 in block_job_detach_aio_context
    #6  0x0000555555a62f4d in bdrv_detach_aio_context
    #7  0x0000555555a63116 in bdrv_set_aio_context
    #8  0x0000555555a9d326 in blk_set_aio_context
    #9  0x00005555557e38da in virtio_blk_data_plane_stop
    #10 0x00005555559f9d5f in virtio_bus_stop_ioeventfd
    #11 0x00005555559fa49b in virtio_bus_stop_ioeventfd
    #12 0x00005555559f6a18 in virtio_pci_stop_ioeventfd
    #13 0x00005555559f6a18 in virtio_pci_reset
    #14 0x00005555559139a9 in qdev_reset_one
    #15 0x0000555555916738 in qbus_walk_children
    #16 0x0000555555913318 in qdev_walk_children
    #17 0x0000555555916738 in qbus_walk_children
    #18 0x00005555559168ca in qemu_devices_reset
    #19 0x000055555581fcbb in pc_machine_reset
    #20 0x00005555558a4d96 in qemu_system_reset
    #21 0x000055555577157a in main_loop_should_exit
    #22 0x000055555577157a in main_loop
    #23 0x000055555577157a in main

The rationale is that the loop in block_job_detach_aio_context cannot
make any progress in pausing/completing the job, because bs->in_flight
is 0, so bdrv_drain doesn't process the block_job_defer_to_main_loop
BH. With this patch, it does.

Reported-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170418143044.12187-3-famz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Tested-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
