provider: Unify default vfunc name
provider: Unify default vfunc name

The default vfunc implementations in GoaProvider are named _real except
for goa_provider_get_provider_icon_default().
This commit changes the "_default" suffix to "_real" for better
consistency.

https://bugzilla.gnome.org/show_bug.cgi?id=752941

felipeborges/gnome-online-accounts,felipeborges/gnome-online-accounts,ritzk/gnome-online-accounts,felipeborges/gnome-online-accounts,ritzk/gnome-online-accounts,ritzk/gnome-online-accounts