hw/arm: Don't fail qtest due to missing SD card in -nodefaults mode

When running omap1/2 or pxa2xx based ARM machines with -nodefaults,
they bail out immediately complaining about a "missing SecureDigital
device". That's not how the "default" devices in vl.c are meant to
work - it should be possible for a board to also start up without
default devices. So let's turn the error message and exit() into
a warning instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 1525326811-3233-1-git-send-email-thuth@redhat.com
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>