curl: implement .bdrv_detach/attach_aio_context()

The curl block driver uses fd handlers, timers, and BHs.  The fd
handlers and timers are managed on behalf of libcurl, which controls
them using callback functions that the block driver implements.

The simplest way to implement .bdrv_detach/attach_aio_context() is to
clean up libcurl in the old event loop and initialize it again in the
new event loop.  We do not need to keep track of anything since there
are no pending requests when the AioContext is changed.

Also make sure to use aio_set_fd_handler() instead of
qemu_aio_set_fd_handler() and aio_bh_new() instead of qemu_bh_new() so
the current AioContext is passed in.

Cc: Alexander Graf <agraf@suse.de>
Cc: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
