net: Don't use error_is_set() to suppress additional errors

Using error_is_set(errp) that way can sweep programming errors under
the carpet when we get called incorrectly with an error set.

qmp_query_rx_filter() breaks its loop when it detects an error.  It
needs to set another error when the loop completes normally.

Return right away instead of merely breaking the loop.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
