[core] Increase media refcount in metadata()
[core] Increase media refcount in metadata()

Even in the case of creating a new media, increase its refcount.

User is in charge of unref it if the media is of non interest for him.

grilofw/grilo,grilofw/grilo,MathieuDuponchelle/grilo,MathieuDuponchelle/grilo,grilofw/grilo,jasuarez/grilo,jasuarez/grilo,jasuarez/grilo,kyoushuu/grilo,GNOME/grilo,kyoushuu/grilo,GNOME/grilo,MathieuDuponchelle/grilo,kyoushuu/grilo,kyoushuu/grilo