Short helper to add a 64 bit counter value to the MIB subtree.

def add_cnt_64bit(self,oid,value,label=None):
		"""Short helper to add a 64 bit counter value to the MIB subtree."""
		# Truncate integer to 64bits ma,x
		self.add_oid_entry(oid,'Counter64',int(value)%18446744073709551615,label=label)