Introduce del_boot_device_path() to clean up fw_cfg content when
hot-unplugging a device that refers to a bootindex or update a
existent devcie's bootindex.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Chenliang <chenliang88@huawei.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>