datapaths with big dpid will cause an overflow

Attempting to create a datapath with dpid larger than `0x7fffffffffffffff` will cause an overflow.

e.g.
```
% vnctl datapaths add --uuid=dp-test --display-name=test --dpid=0xdeadbeefdeadbeef --node-id=vna
```
This generates `:dpid: '0x7fffffffffffffff'`.

The type of `DATAPATHS.dpid` should be `unsigned`?
