* Jonathan meets governors over growing Boko Haram violence
* Bomb blast hit Abuja, 200 abducted girls still missing
* Violence also on the rise in religiously mixed Middle Belt
By Felix Onuah
ABUJA, April 24 (Reuters) - Leaders from across Nigeria appealed for a united front against Boko Haram on Thursday, saying the Islamist insurgents were waging war on Christian and Muslim Nigerians...