datapath-windows: NUL character should be left out during VPORT hash lookup

While calculating the hash on a VPORT name, we don't include the NUL character.
We should be doing the same while doing lookup as well.

We set the required minimum length of the name to be 2 so that the string has
at least one valid character.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Acked-by: Eitan Eliahu <eliahue@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>