Drop the assumption that we're using the main AioContext. Convert
qemu_bh_new() to aio_bh_new() and qemu_aio_wait() to aio_poll() so we
use the BlockDriverState's AioContext.
Implement .bdrv_detach/attach_aio_context() interfaces to propagate
detach/attach to BDRVBlkverifyState->test_file. The block layer takes
care of ->file and ->backing_hd but doesn't know about our ->test_file
BlockDriverState, which is also part of the graph.
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>