vfio/pci: Clear error and request eventfd ctx after releasing

The next use of the device will generate an underflow from the
stale reference.

Cc: Qian Cai <cai@lca.pw>
Fixes: 1518ac27 ("vfio/pci: fix memory leaks of eventfd ctx")
Reported-by: Daniel Wagner <dwagner@suse.de>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Tested-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>