Add helpers to better structure CPU option handling

Add helpers to better structure CPU option handling

To avoid hardcoding even more CPU-flag related things for custom CPU
models, introduce a dynamic approach to resolving flags.

resolve_cpu_flags takes a list of hashes (as documented in the
comment) and resolves them to a valid "-cpu" argument without
duplicates. This also helps by providing a reason why specific CPU flags
have been added, and thus allows for useful warning messages should a
flag be overwritten by another.

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>