usb: tag usb host adapters as not hotpluggable.

Hotplugging them simply doesn't work, so tag them accordingly to
avoid users trying and then crashing qemu.

For xhci there is nothing fundamental which prevents hotplug from
working, we'll "only" need a exit() function which cleans up
everything properly.  That isn't for 1.3 though.

For ehci+uhci+ohci hotplug can't be supported until qemu gains the
capability to hotplug multifunction pci devices.

https://bugzilla.redhat.com/show_bug.cgi?id=879096

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