virsh.cpu_baseline: Using baseline cpu to config guest

Signed-off-by: Yanbing Du <ydu@redhat.com>
Not really, most tests in this case are not need VM, that's why ```vms``` and ```main_vm``` are set to empty by default, so only get VM when test need.
And these two lines not included in ```try...finally``` blcok, most cases deal with it in this way.