Fix detection of physical volumes

The detection of physical volumes does not work at Debian Wheezy and Ubuntu Quantal (Ruby 1.9.3), because the result is a string instead of an array.

```
Debug: Executing '/sbin/pvs /dev/vda5'
Debug: Executing '/sbin/vgs vg0'
Debug: Executing '/sbin/pvs -o pv_name,vg_name --separator ,'
Error: /Stage[main]//Node[default]/Lvm::Volume[swap]/Volume_group[vg0]: Could not evaluate: undefined method `inject' for "  PV,VG\n  /dev/vda5,vg0\n":String
```