virsh.cpu_baseline: Using baseline cpu to config guest

Signed-off-by: Yanbing Du <ydu@redhat.com>
Hard code for this main_vm is not convenient.
You can put vms/main_vm in to other parameter temporarily at the beginning of reset:
```
this_vms = ${vms}
vms = ""
main_vm = ""
```

Then for config_guest:
```
vms = ${this_vms}
```
So as main_vm.