sheepdog: implement .bdrv_detach/attach_aio_context()

sheepdog: implement .bdrv_detach/attach_aio_context()

Drop the assumption that we're using the main AioContext.  Convert
qemu_aio_set_fd_handler() to aio_set_fd_handler() and qemu_aio_wait() to
aio_poll().

The .bdrv_detach/attach_aio_context() interfaces also need to be
implemented to move the socket fd handler from the old to the new
AioContext.

Cc: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Acked-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>