[RFC] xen: Add a "floppy" type, alongside cdrom and hard disk (take #2)

Original pull request: https://github.com/xapi-project/xcp-idl/pull/46
Updated with a new floppy bus type, as qemu wants floppy drives to be on the "floppy" bus, rather than "ide".

Note that this means that `Device_number.to_disk_number` is no longer unique, and should not be used to identify disks. [This is the subject of a bugfix in xen-api -- see pull request]