ui: lxc/qemu: cpu edit: use emptyText for cpuunits

ui: lxc/qemu: cpu edit: use emptyText for cpuunits

This makes it slightly clearer that it's a default value (not being
written to the config explicitly). It's also in preparation to bind
the default based on the cgroup version, where it's a bit nicer to
bind the emptyText instead of the value (or a user set value would be
overwritten when the binding updates).

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