After g_source_attach() the GMainContext holds a reference to the
GSource, so the caller does not need to keep it.

qio_task_thread_worker() is not releasing its reference so the GSource
is being leaked since a17536c5.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <1565625509-404969-2-git-send-email-andrey.shinkevich@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>