ssh: use BlockDriverState's AioContext

ssh: use BlockDriverState's AioContext

Drop the assumption that we're using the main AioContext.  Use
bdrv_get_aio_context() to register fd handlers in the right AioContext
for this BlockDriverState.

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.

For now this doesn't make much difference but will allow ssh to work in
IOThread instances in the future.

Acked-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>