Allow the user to specify target filenames of cloned disk drives.

Hi,

I often clone instances from "template" VMs and would like to have more control of the image names to be able to tell them apart. When cloing, for example, "debian-template" with the disk volume "debian-template.img", the target file name is always "debian-template-clone.img" today. I'd like to be able to choose the file name of the new image file, and not just the instance name, if that makes sense.

This patch makes the target file names editable, and generates the new XML file with that in mind. The file names default to the current standard, like "oldfile-clone.img". It works fine on my system, running qemu, even with more than one storage device, but I'd recommend you to test this a bit first before committing it to master. Anyway, I hope it's a step in the right direction towards more complete user control.