Teach get-vm-ip to wait for the VM.

We now wait indefinitely for the seed VM to boot and its IP to become
available. This is useful for manual runs of devtest.md, but is vital
for runs of devtest.sh.

Change-Id: If60fe9fd751e3d3faca65108a9cba08e0168b9f5
Merge "Teach get-vm-ip to wait for the VM."