ide: add calling add_boot_device_patch in bootindex setter function

On this way, we can assure the new bootindex take effect
during vm rebooting. Meanwhile set the initial value of
bootindex to -1.

Because ide devcies's unit property maybe
do not initialize when set_bootindex function is called,
so that we don't know its suffix. So we have to save the
call add_boot_device_path() on ide realize/init function.
When we want to change bootindex during vm rebooting, we
can call it in setter function.

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