api: create/update vm: clamp cpuunit value

api: create/update vm: clamp cpuunit value

While the clamping already happens before setting the actual systemd
CPU{Shares, Weight}, it can be done here too, to avoid writing new
out-of-range values into the config.

Can't use a validator enforcing this because existing out-of-range
values should not become errors upon parsing the config.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>