block: use aio_bh_schedule_oneshot

This simplifies bottom half handlers by removing calls to qemu_bh_delete and
thus removing the need to stash the bottom half pointer in the opaque
datum.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>