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

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 18b6dad
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: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel.a@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>