Drop the assumption that we're using the main AioContext for Linux
AIO. Convert qemu_aio_set_fd_handler() to aio_set_fd_handler() and
timer_new_ms() to aio_timer_new().
The .bdrv_detach/attach_aio_context() interfaces also need to be
implemented to move the fd and timer from the old to the new AioContext.
Cc: Peter Lieven <pl@kamp.de> Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Peter Lieven <pl@kamp.de>