Adapt CPUConfig to handle custom models

Adapt CPUConfig to handle custom models

Turn CPUConfig into a SectionConfig with parsing/writing support for
custom CPU models. IO is handled using cfs.

Namespacing will be provided using "custom-" prefix for custom model
names (in VM config only, cpu-models.conf will contain unprefixed
names).

Includes two overrides to avoid writing redundant information to the
config file, additionally get_custom_model is used to retrieve a custom
model configuration by name.

Resolve custom names in print_cpu_device when a custom cpu is passed.

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