qdev: Avoid type assertion in qdev_build_hotpluggable_device_list()

Currently when *obj is not a TYPE_DEVICE, QEMU will abort. This patch
fixes it. When *obj is not a TYPE_DEVICE, just do not add it to hotpluggable
device list.

This patch also fixes the following issue:
1. boot QEMU using cli:
$ /opt/qemu-git-arm/bin/qemu-system-x86_64 -monitor stdio -enable-kvm \
-device virtio-scsi-pci,id=scsi0

2. device_del scsi0 via hmp using tab key(first input device_del, then press
"Tab" key).
(qemu) device_del

After step 2, QEMU will abort.
(qemu) device_del hw/core/qdev.c:930:qdev_build_hotpluggable_device_list:
Object 0x5555563a2460 is not an instance of type device

Signed-off-by: Jun Li <junmuzi@gmail.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Andreas Färber <afaerber@suse.de>