fix #2671: include CPU format in man page again

fix #2671: include CPU format in man page again

Use the new register_format(3) call to use a validator (instead of a
parser) for 'pve-(vm-)?cpu-conf'. This way the $cpu_fmt hash can be used for
generating the documentation, while still applying the same verification
rules as before.

Since the function no longer parses but only verifies, the parsing in
print_cpu_device/get_cpu_options has to go via JSONSchema directly.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>