Add get/set for gvcp packet count value.
Add get/set for gvcp packet count value.

lu-zero/aravis,lu-zero/aravis,AnilRamachandran/aravis,AnilRamachandran/aravis,lu-zero/aravis,AravisProject/aravis,AravisProject/aravis,AnilRamachandran/aravis,lu-zero/aravis,lu-zero/aravis,AnilRamachandran/aravis,AravisProject/aravis,AravisProject/aravis,AravisProject/aravis,AnilRamachandran/aravis