qdev: Fix crash when device_add is called with abstract driver

User is able to crash running QEMU when following monitor
command is called:

 device_add intel-hda-generic

Crash is caused by assertion in object_initialize_with_type()
when type is abstract.

Checking if type is abstract before instance is created in
qdev_device_add() allows to prevent crash on incorrect user input.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>