qdev: Use qdev_get_device_class() for -device <type>,help

Make sure we try to list properties from classes that can be safely used
with "-device".

Fixes the following crashes:

  $ qemu-system-x86_64 -device x86_64-cpu,help
  **
  ERROR:qom/object.c:336:object_initialize_with_type: assertion failed: (type->abstract == false)
  Aborted (core dumped)
  $ qemu-system-x86_64 -device host-x86_64-cpu,help
  qemu-system-x86_64: [...]/target-i386/cpu.c:1329: host_x86_cpu_initfn: Assertion `(kvm_allowed)' failed.
  Aborted (core dumped)

After applying this patch:

  $ qemu-system-x86_64 -device x86_64-cpu,help
  Parameter 'driver' expects non-abstract device type
  $ qemu-system-x86_64 -device host-x86_64-cpu,help
  Parameter 'driver' expects pluggable device type

Signed-off-by: Eduardo Habkost <[email protected]>
Signed-off-by: Andreas Färber <[email protected]>

Author: ehabkost

qdev-monitor.c

@@ -235,12 +235,9 @@ int qdev_device_help(QemuOpts *opts)
         return 0;
     }
 
-    if (!object_class_by_name(driver)) {
-        const char *typename = find_typename_by_alias(driver);
-
-        if (typename) {
-            driver = typename;
-        }
+    qdev_get_device_class(&driver, &local_err);
+    if (local_err) {
+        goto error;
     }
 
     prop_list = qmp_device_list_properties(driver, &local_err);