vrouter: Fixing problem with sending NLMSG_DONE from vr_transport_request

There has been problem with improperly prepared mbuf for NLMSG_DONE,
which prevented the message from being send properly.
The problem was that the size of message was not set properly.
Merge pull request #3 from Semihalf/fix-vrouter-nlmsg-done

vrouter: Fixing problem with sending NLMSG_DONE from vrouter module