virsh.cpu_baseline: Using baseline cpu to config guest

Signed-off-by: Yanbing Du <ydu@redhat.com>
if most tests don't need VM, it's an issue in your 'finally' section, it means 'vm' object is empty then can't call is_alive() method, as I said in above comment,  a safe method is check if 'vm' object exist firstly like so:
if vm and vm.alive():
    xxxx