systemd: fix build without dbus
    
    The virDBusMethodCall method has a DBusError as one of its
    parameters. If the caller wants to pass a non-NULL value
    for this, it immediately makes the calling code require
    DBus at build time. This has led to breakage of non-DBus
    builds several times. It is desirable that only the virdbus.c
    file should need WITH_DBUS conditionals, so we must ideally
    remove the DBusError parameter from the method.
    
    We can't simply raise a libvirt error, since the whole point
    of this parameter is to give the callers a way to check if
    the error is one they want to ignore, without having the logs
    polluted with an error message. So, we add a virErrorPtr
    parameter which the caller can then either ignore or raise
    using the new virReportErrorObject method.
    
    This new method is distinct from virSetError in that it
    ensures the logging hooks are run.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>