Originally we were storing the GSources tag IDs.  That'll be not enough
if we are going to support non-default gcontext for QIO code.  Switch to
GSources without changing anything real.  Now we still always pass in
NULL, which means the default gcontext.

Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>