hw/usb/dev-serial: Do not try to set vendorid or productid properties

When starting QEMU with the legacy USB serial device like this:

 qemu-system-x86_64 -usbdevice serial:vendorid=0x1234:stdio

it currently aborts since the vendorid property does not exist
anymore (it has been removed by commit f29783f7):

 Unexpected error in object_property_find() at qemu/qom/object.c:1008:
 qemu-system-x86_64: -usbdevice serial:vendorid=0x1234:stdio: Property
                     '.vendorid' not found
 Aborted (core dumped)

Fix this crash by issuing a more friendly error message instead
(and simplify the code also a little bit this way).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 1493883704-27604-1-git-send-email-thuth@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>