Feat: Add CPU model and topology support

[NamelessOne91]

Related to:

• Feature request; CPU topology, sockets, cores, threads #686
• xml parameter or match, model, check are not available under cpu block for a domain #1119
• set custom cpu model - to enable kvm live migration #1020

Adds support for defining model and topology blocks on a libvirt domain's CPU.
This is possible with 2 limitations:

• to define a model, the CPU mode must be set to "custom"
• sockets * threads * cores must be <= to the number of vCPU

Example:

resource "libvirt_domain" "test-domain" {
    name = "test"
    vcpu = 4
    cpu  {
        mode = "custom"
        model {
            fallback = "forbid"
            value = "EPYC"
            vendor_id = "AuthenticAMD"
        }
        topology {
            sockets = 1
            cores = 2
            threads = 2
        }
    }
}

[bradthebuilder]

I wasn't able to get a aarch64 VM to spawn until I tried using this PR. Worked like a charm!

[localghost]

@dmacvicar any plans to iterate "over interesting PRs and do minor improvements" (specifically this one) in the near future?