Rework get_cpu_options and allow custom CPU models

Rework get_cpu_options and allow custom CPU models

If a cputype is custom (check via prefix), try to load options from the
custom CPU model config, and set values accordingly.

While at it, extract currently hardcoded values into seperate sub and add
reasonings.

Since the new flag resolving outputs flags in sorted order for
consistency, adapt the test cases to not break. Only the order is
changed, not which flags are present.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
Reviewed-By: Fabian Ebner <f.ebner@proxmox.com>
Tested-By: Fabian Ebner <f.ebner@proxmox.com>