Remove broken error reporting in QEMU mac filtering

Remove broken error reporting in QEMU mac filtering

The qemu_bridge_filter.c file had some helpers for calling
the ebtablesXXX functions todo bridge filtering. The only
thing these helpers did was to overwrite the original error
message from the ebtables code. For added fun, the callers
of these helpers overwrote the errors yet again. For even
more fun, one of the helpers called another helper and
overwrite its errors too.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>