Cross-Hypervisor Image Metadata

Offer an option for the creator of an image to express the hardware compatibility of the image in a hypervisor independent way.
Fields such as `hw_disk_bus` and `hw_vif_model` allow the creator of an image to choose a hardware model, but the choice depends on the knowledge of the hardware supported by the hypervisor and sometimes even version. On the other hand, the supported hardware depends on the drivers installed in the image. This proposal presents a new way of negotiating between the creator of and the driver implementation in a hypervisor independent way.