qom: Avoid leaking str and bool properties on failure

When object_property_add_str() and object_property_add_bool() fail, they
leak their internal StringProperty and BoolProperty structs.  Remember
to free the structs on error.

Luckily this is a low-impact memory leak since most QOM properties are
static qdev properties that will never take the error case.
object_property_add() only fails if the property name is already in use.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Andreas Färber <afaerber@suse.de>