[libvirt] Improve disk bus type generation

This is mostly an excuse to add a _get_supported_devices() method used in fallback cases (when the deployment does not specify a disk driver to be used) as this will be useful later. The list of supported devices is generated with - devices supported both by the OS and the platform (hypervisor) - devices for which we have a OS driver, and which are supported by the platform When doing this work, I realized a few things were missing from libosinfo: - to properly guess the bus to use for a disk (sata, ide, scsi, ...) from a libosinfo device, we'd need to have the PCI subclass in libosinfo. This will probably need to be added at some point - something that would be nice for easier fallback when the deployment does not specify a device would be a way for OS/platforms to indicate that their preferred disk device is virtio-disk, then sata device XXX, ... I also think deployments would be more flexible if a deployment meant for qemu-1.2.0 could be used with qemu-1.3.0. Currently, osinfo_db_find_deployment does not follow 'upgrades' relations on platforms, but I think it would be more convenient if it did. Christophe