char: Fix removing wrong GSource that be found by fd_in_tag

We use fd_in_tag to find a GSource, fd_in_tag is return value of
g_source_attach(GSource *source, GMainContext *context), the return
value is unique only in the same context, so we may get the same
values with different 'context' parameters.

It is no problem to find the right fd_in_tag by using
 g_main_context_find_source_by_id(GMainContext *context, guint source_id)
while there is only one default main context.

But colo-compare tries to create/use its own context, and if we pass wrong
'context' parameter with right fd_in_tag, we will find a wrong GSource to handle.
We tried to fix the related codes in commit b43decb0,
but it didn't fix the bug completely, because we still have some codes didn't pass
*right* context parameter for remove_fd_in_watch().

Let's fix it by record the GSource directly instead of fd_in_tag.

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1492564532-91680-1-git-send-email-zhang.zhanghailiang@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>