Calling error_report() in a function that takes an Error ** argument
is suspicious.  add_channel() does that, and then exit()s.  Its caller
main(), via qemu_opts_foreach(), is fine with it, but clean it up
anyway.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20181017082702.5581-31-armbru@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>