Make CreateVnodes() a bit more robust.
Make CreateVnodes() a bit more robust.

In particular, if $options->{'vlist'} is not present or too short,
do not attempt to look up any virt_node_attributes.

nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome