Fix ChannelMapping subobj parsing in VDSObjectParser
The ChannelMapping sub-obj type is N64 (Int64), not U64 (UInt64), change to avoid jsoncpp exception when parsing a value of -1 which can occur in some VDSs.
The ChannelMapping sub-obj type is N64 (Int64), not U64 (UInt64), change to avoid jsoncpp exception when parsing a value of -1 which can occur in some VDSs.