monitor-config: ignore stored config when hotplug_mode_update is set

When the output device has hotplug_mode_update (e.g. the qxl driver used in
vms), the displays can be dynamically resized, so the current display
configuration does not often match a stored configuration. When a new
monitor is added, make_default_config() tries to create a new display
configuration by choosing a stored configuration with N-1 monitors, and then
adding a new monitor to the end of the layout. Because the stored config
doesn't match the current outputs, apply_configuration() will routinely
fail, leaving the additional display unconfigured. In this case, it's more
useful to just fall back to creating a new default configuration from
scratch so that all outputs get configured to their preferred mode.