[libvirt] Add GVirConfigDomainDiskDriver

Hey, I wanted to add support in libvirt-gconfig for the 'discard' attribute of the disk driver node. If I follow the way the API is currently done, it would be an additional method to GVirConfigDomainDisk. However, there are quite a few attributes attached to the disk driver node, so I felt it was preferrable to have a dedicated GVirConfigDomainDiskDriver class. This also matches better other places of libvirt-gconfig API. I've implemented support for most of the attributes of the disk driver node, as a result the corresponding methods in GVirConfigDomainDisk have been deprecated. Christophe