qemu: Reject unsupported tuning in session mode
    
    When domain is started with setting that cannot be done, i.e. those
    that require cgroups, there is no error reported and it succeeds
    without any message whatsoever.
    
    When setting with API, virsh, an error is reported, but only due to
    the fact that no cgroups are mounted (priv->cgroup == NULL).
    
    Given the above it seems reasonable to reject such unsupported
    settings.
    
    This patch effectively changes the error message from:
    
    $ virsh -c qemu:///session schedinfo dummy
    Scheduler      : Unknown
    error: Requested operation is not valid: cgroup CPU controller is not mounted
    
    to:
    
    $ virsh -c qemu:///session schedinfo dummy
    Scheduler      : Unknown
    error: Operation not supported: CPU tuning is not available in session mode
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1023366
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>