virsh.cpu_compare: Case update and bug fix

1. Add a host feature to VM if it has no cpu feature, avoid skip related
cases.
2. For 'host-model', as libvirt does not model every aspect of each CPU,
so the guest CPU will not match the host CPU exactly. So, after start
guest, just check if the new add cpu features of guest exist on host.
3. Always re-define VM after case finished.

Signed-off-by: Yanbing Du <ydu@redhat.com>
Merge pull request #368 from will-Do/cpu_compare_fix

virsh.cpu_compare: Case update and bug fix