make usb devices configurable

make usb devices configurable

Leave the core usb devices (usb hub, tablet, mouse, keyboard)
enabled unconditionally.  Make the other ones configurable.

Exceptions:
  - bluetooth: not qdevified yet, has a vl.c dependency because
    of that, thus disabling isn't as easy as not linking the
    object file.
  - smardcard: ccid-card-emulated depends on that one *and*
    CONFIG_SMARTCARD_NSS.  So it isn't a one-liner and comes
    as separate patch because of that.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>