Allow specifying fixed IPs for new instances

It is possible to specify a VMs network interfaces using port IDs.  This
allows for predetermining the IP address a VM will use in a subnet.

  --fixed-ips 0fba3583-7f74-49f8-aa1d-8746fee7479f=10.0.0.54

The code takes care of the situation where an unused port with that IP address in that subnet already exists and takes that one.