api: create/update vm: clamp cpu unit value

api: create/update vm: clamp cpu unit value

While the clamping already happens before setting the actual
cpu.weight lxc config key, 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 on parsing the config.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>