qemu: Ignore non-boolean CPU model properties
    
    The query-cpu-model-expansion is currently implemented for s390(x) only
    and all CPU properties it returns are booleans. However, x86
    implementation will report more types of properties. Without making the
    code more tolerant older libvirt would fail to probe newer QEMU
    versions.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>