fixes 3177: a 'G' suffix is automatically appended to libvirt volume capacity value if none was specified

I think the UI to volume capacity could be improved. libvirt internally stores volume sizes in bytes, but allows to specify the size in a variety of units. We could switch to follow the same convention. 

It's a bit confusing atm, as libvirt expects a suffix to be present, but we only support one -- "G".