cpus: host option in template files

### Description

It would be nice to have a `cpus: host` option that automatically uses all host cores. (similar to QEMU option)

Or perhaps to be even more flexible: a `cpus: host-2` or `cpus: performance` option. Where `host-2` means the number of host CPUs minus 2 and `performance` means use the the (e.g. 8) host CPU "performance" cores, but leave the (e.g. 2) host "efficiency" cores free for the host OS.

Options like these could make the default templates much more useful.