hw: Don't call visit_end_struct() after visit_start_struct() fails

hw: Don't call visit_end_struct() after visit_start_struct() fails

When visit_start_struct() fails, visit_end_struct() must not be
called.  rtc_get_date() and balloon_stats_all() call it anyway.  As
far as I can tell, they're only used with the string output visitor,
which doesn't care.  Fix them anyway.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>