qdev: add no_user, alias and desc

no_user: prevent users from adding certain devices.
desc: description of the device.
alias: to allow user friendly shortcuts on the command line, i.e.
  -device usbmouse  instead of  -device "QEMU USB Mouse"  or
  -device lsi       instead of  -device lsi53c895a

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
