Add affordance for CPUID leaves to be set by instance initialization

[gjcolombo]

propolis::vcpu::Vcpu::set_cpuid lets callers set the values the CPUID instruction should return when a guest executes it on the target vCPU. Standalone can make use of this API, but currently Server cannot. There should be some way to specify CPUID profiles when creating Propolis server VMMs. (This ties into the broader work needed to define virtual platforms in the control plane, which is needed to support control plane-directed live migration during upgrade.)