add: new config option for virt module

Hi,

I am using XEN as a dom0 on some of my servers and the *virt* module wasn't able to retrieve some informations like the running VM.

Although I set the ```uri_default``` option in my ```/etc/libvirt/libvirt.conf``` this parameter did not seem to be used by the module.

So I wrote this little patch that allows us to set the proper *connect string* in the minion's configuration file.

This new option is named ```virt.connect```

Cheers