iothread: push gcontext earlier in the thread_fn

We were pushing the context until right before running the gmainloop.
Now since we have everything unconditionally, we can move this
earlier.

One benefit is that now it's done even before init_done_sem, so as
long as the iothread user calls iothread_create() and completes, we
know that the thread stack is ready.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-id: 20190306115532.23025-5-peterx@redhat.com
Message-Id: <20190306115532.23025-5-peterx@redhat.com>

[Tweaked comment wording as discussed with Peter Xu.
--Stefan]

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>