vfio/pci: Clear token on bypass registration failure

The eventfd context is used as our irqbypass token, therefore if an
eventfd is re-used, our token is the same.  The irqbypass code will
return an -EBUSY in this case, but we'll still attempt to unregister
the producer, where if that duplicate token still exists, results in
removing the wrong object.  Clear the token of failed producers so
that they harmlessly fall out when unregistered.

Fixes: 6d7425f1 ("vfio: Register/unregister irq_bypass_producer")
Reported-by: guomin chen <guomin_chen@sina.com>
Tested-by: guomin chen <guomin_chen@sina.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>