Choose -cpu max by default

Choose -cpu max by default

You must set -cpu to something on RHEL 10 with TCG since the kernel on
that platform now requires x86_64-v3.  Choosing a good default is
difficult.  Use -cpu max as that was supposed to fix this, although in
practice it doesn't work very reliably.

This is likely a temporary solution.  More reliable/permanent
solutions include:

 - Allowing the user to set the default CPU at configure time.

 - A large case statement that tries to get it right for every
   possible combination of platform/architecture/KVM/...

 - Fixing -cpu max in qemu