seccomp: Clean up error reporting in parse_sandbox()

seccomp: Clean up error reporting in parse_sandbox()

Calling error_report() in a function that takes an Error ** argument
is suspicious.  parse_sandbox() does that, and then fails without
setting an error.  Its caller main(), via qemu_opts_foreach(), is fine
with it, but clean it up anyway.

Cc: Eduardo Otubo <otubo@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Eduardo Otubo <otubo@redhat.com>
Message-Id: <20181017082702.5581-18-armbru@redhat.com>