vl: Clean up error reporting in machine_set_property()

vl: Clean up error reporting in machine_set_property()

Calling error_report() in a function that takes an Error ** argument
is suspicious.  machine_set_property() 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.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20181017082702.5581-22-armbru@redhat.com>