qdev: Do not let the user try to device_add when it cannot work

Such devices have always been unavailable and omitted from the list of
available devices shown by device_add help.  Until commit 18b6dade
silently broke the former, setting up nasty traps for unwary users,
like this one:

    $ qemu-system-x86_64 -nodefaults -monitor stdio -display none
    QEMU 1.6.50 monitor - type 'help' for more information
    (qemu) device_add apic
    Segmentation fault (core dumped)

I call that a regression.  Fix it.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NMarcel Apfelbaum <marcel.a@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>

qdev-monitor.c

@@ -490,6 +490,11 @@ DeviceState *qdev_device_add(QemuOpts *opts)
     }
 
     dc = DEVICE_CLASS(oc);
+    if (dc->cannot_instantiate_with_device_add_yet) {
+        qerror_report(QERR_INVALID_PARAMETER_VALUE, "driver",
+                      "pluggable device type");
+        return NULL;
+    }
 
     /* find bus */
     path = qemu_opt_get(opts, "bus");