use helper from common for cpu units/shares

use helper from common for cpu units/shares

to make behavior more consistent with what we do for VMs. The helper
will clamp the value as needed, rather than dying.

Allows starting existing containers with an out-of-range (for the
relevant cgroup version) value. It's also possible to end up with
out-of-range values via update/create API.

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