virsh.cpu_baseline: Using baseline cpu to config guest

Signed-off-by: Yanbing Du <ydu@redhat.com>
I think it should be a desgin issue in test framework, if you have multiple VMs you need 'vms' parameter, otherwise,
the 'main_vm' should be enough, but for now, if you don't need any VMs, you must set vms="" and main_vm="" together, okay, I know this.