qemu: Add KVM CPUs into cache only if KVM is present

qemu: Add KVM CPUs into cache only if KVM is present

virQEMUCapsFormatCache/virQEMUCapsLoadCache adds/reads KVM CPUs to/from
capabilities cache regardless of QEMU_CAPS_KVM. That can cause undesired
side-effects when KVM CPUs are present in the cache on a platform that
doesn't support it, e.g. macOS or Linux without KVM support.

Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Tested-by: Brad Laue <brad@brad-x.com>
Tested-by: Christophe Fergeau <cfergeau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>