Is libvirt the lowest common denominator of hypervisor features?
Short answer: ABSOLUTELY NOT. Libvirt’s goal is to expose all useful hypervisor features. Period. The only caveat is that this feature needs to be exposed in a general way that is compatible with the libvirt architecture. Even if only a single supported hypervisor implements feature FOO, the API and XML changes need to be made sufficiently general in case any other hypervisor eventually supports FOO.