hw/qdev-clock: Display error hint when clock is missing from device

hw/qdev-clock: Display error hint when clock is missing from device

Instead of directly aborting, display a hint to help the developer
figure out the problem (likely trying to connect a clock to a device
pre-dating the Clock API, thus not expecting clocks).

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Reviewed-by: Damien Hedde <damien.hedde@greensocs.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-Id: <20201012095804.3335117-4-f4bug@amsat.org>