libvirt: Fix nova-compute start when missing ip.

If nova-compute is restarted when an instance has no ip address
the libvirt/vif:_get_configuration method will throw an index
error. Check for existance of an ip before attempting to retrieve
one. Includes failing test.

Fixes bug 1131330

Change-Id: Id383544b44e64205fc3b4f850d0d11ad2ebd5da7