ssi-sd: Make devices picking up backends unavailable with -device

ssi-sd: Make devices picking up backends unavailable with -device

Device models aren't supposed to go on fishing expeditions for
backends.  They should expose suitable properties for the user to set.
For onboard devices, board code sets them.

Device ssi-sd picks up its block backend in its init() method with
drive_get_next() instead.  This mistake is already marked FIXME since
commit af9e40a.

Unset user_creatable to remove the mistake from our external
interface.  Since the SSI bus doesn't support hotplug, only -device
can be affected.  Only certain ARM machines have ssi-sd and provide an
SSI bus for it; this patch breaks -device ssi-sd for these machines.
No actual use of -device ssi-sd is known.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Acked-by: Thomas Huth <thuth@redhat.com>
Message-id: 20181009060835.4608-1-armbru@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>