virsh.cpu_baseline: Using baseline cpu to config guest

Signed-off-by: Yanbing Du <ydu@redhat.com>
Cases don't need VM will not enter ```if config_guest and status == 0:``` block, and here the ```try ... finally ... ``` block inside the if condition is for VM related tests, 'vm' object will not None.(Of course, we can't make sure it is not None all the time, but then throw the corresponding Exception is useful and acceptable)