Fix broken libvirt recipe test

As it stands right now, libvirt calls out to system to execute
grub2-set-default and this was stubbed out only in the Suse chefspec
libvirt test. Move this stub to spec_helper and modify our tailor
configuration to allow for this change.

Change-Id: I276d799cb3db5f725ffa396b5bed8724b6e42696