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 <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>