I received a report that vif-bridge adds any tap interface to a bridge,
regardless if xen is running and who created the tap interface.  E.g.

# tunctl -p -t tap42

will cause vif-bridge to be executed as per the following rule in
xen-backend.rules

SUBSYSTEM=="net", KERNEL=="tap*", ACTION=="add",
RUN+="/etc/xen/scripts/vif-setup $env{ACTION} type_if=tap"

I'm not sure how to improve the rule to prevent execution of vif-setup
in this case.  But it seems better to handle it in vif-bridge anyhow, by
not connecting the interface to a bridge if there is no corresponding
info in xenstore.  Something along the lines of the attached quick
patch.  Comments?

Thanks!
Jim