Drop the assumption that we're using the main AioContext. Convert
qemu_bh_new() to aio_bh_new() so we use the BlockDriverState's
AioContext.
The .bdrv_detach_aio_context() and .bdrv_attach_aio_context() interfaces
are not needed since no fd handlers, timers, or BHs stay registered when
requests have been drained.
Cc: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>