Warn if value of qdev_init() isn't checked

After qdev_init() fails, the device is gone.  Failure to check runs a
high risk of use-after-free.

Patchworks-ID: 35166
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
