[libvirt] Add GVirConfigDomainDiskDriver

[Christophe Fergeau]

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