wire: add VirtualSize method to MsgTx
wire: add VirtualSize method to MsgTx

Punting on resolving the import cycle. VirtualSize _wants_ to use
constants currently defined in blockchain, but, that would cause an
import cycle. Options:
  1. move VirtualSize as fun to blockchain
  2. re-define the consensus critical constants in wire

adiabat/btcd,adiabat/btcd