GIR writer: Fix virtual method generation
GIR writer: Fix virtual method generation

Generate virtual and abstract methods correctly (method +
virtual-method + callback in type struct).

Signed-off-by: Didier 'Ptitjes <9b75d9bd0301369bd978ce18252ab1844af4cc4b@free.fr>

kamanashisroy/aroop,spidermonkdust/vala,flobrosch/vala-fixes,themiwi/vala,severinh/vala,flobrosch/vala-fixes,Jc2k/vala,jessevdk/vala,Distrotech/vala,Jc2k/vala,GNOME/vala,frida/vala,frida/vala,Distrotech/vala,tjol/vala-tjol,lovebug356/vala,Distrotech/vala,spidermonkdust/vala,frida/vala,lovebug356/vala,frida/vala,themiwi/vala,jessevdk/vala,GNOME/vala,kamanashisroy/aroop,playya/vala,kamanashisroy/aroop,severinh/vala,phako/vala,flobrosch/vala-fixes,GNOME/vala,themiwi/vala,phako/vala,spidermonkdust/vala,jessevdk/vala,rvu95/vala,GNOME/vala,severinh/vala,frida/vala,rvu95/vala,playya/vala,xdave/pvala,tjol/vala-tjol,playya/vala,rvu95/vala,tjol/vala-tjol