installer:fixed unmount dialog show

description:
    when in pxe, and switch to advanced mode, only the swap part has mounted,
    and then the unmount dialog should not show out

tower:https://tower.im/projects/8f19f0bf0e754f0b82ef2c24bc230973/todos/45ce6d3f58474bc79182605fcbe72bf7/

Change-Id: I7dc59ac417c83bcb4575f251271b8b4db6762958