String-convert IPAddr objects for FixedIP.attach()

This fixes a couple places where we pass a (non-serializeable)
IPAddr object to the FixedIP.attach() method, which blows up.
We probably need a more generic solution to this problem, as it
keeps coming up. However, for the moment, this is less problematic
than the patch reverted just before this.

Change-Id: I716c558a3535b3ed5c9b10f5788a4463a3daa3d3
Merge "String-convert IPAddr objects for FixedIP.attach()"