clone only required "view" access, so a user with clone_instances and the
global view_instances permission could clone anyone's VM and read its
disks. It also copied every POST field into clone_data, which let the
client override the server-set disk_owner_uid/gid, and disk-<dev> values
went unvalidated into volume XML and os.path.join.
- Require change permission on the source VM, except for templates,
which any user who can view them may clone.
- Build clone_data from an allowlist: name, title and description for
everyone; disk names and MAC addresses only from superusers.
- For non-superusers, derive disk names (get_clone_disk_name) and MAC
addresses (DHCP or random) on the server; their form only sent the
source VM's own values, which could never work.
- Validate every MAC address (not only the first) and every disk name;
a missing MAC is an error message instead of a KeyError.
