usb: USBPacket: add status, rename owner -> ep

Add enum to track the status of USBPackets, use that instead of the
owner pointer to figure whenever a usb packet is currently in flight
or not.  Add some more packet status sanity checks.  Also rename the
USBEndpoint pointer from "owner" to "ep".

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