qdev: correctly send DEVICE_DELETED for recursively-deleted devices

When a device is unparented (i.e. made completely hidden from management)
we want to send a DEVICE_DELETED event only if the device actually was
realized.  This avoids raising DEVICE_DELETED events when device_add
fails.

However, this does not work right for recursively-deleted
devices: the whole tree is _first_ unrealized, _then_ unparented.
Then device_unparent sees realized==false and fails to trigger
the event.  The solution is simply to move have_realized into
the DeviceState struct.  If device_add fails, we never set the
new field to true and DEVICE_DELETED is not sent.

Fixes qemu-iotests testcase 067 (broken by commit 5942a19, though that
commit in turn fixed a possible segfault in the same test).

Reported-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
