Fixes some puppet lint warnings

This commit fixes the following warnings:
manifests/compute/libvirt.pp - WARNING: double quoted string containing no variables on line 31
manifests/init.pp - WARNING: class inheriting from params class on line 77
manifests/migration/libvirt.pp - WARNING: case statement without a default case on line 4

Change-Id: I84a1f5edfd63973fcfa963f9acfa954cf06c094b