vt(4): Pause the flush timer while swithing window

This fixes bad looking refresh when switching window: squares instead
of text, flashing screen, and so on. In the worst case, vt_flush() came
at a very inappropriate timing and the screen was not refreshed at all
(leaving squares all over the place).

This doesn't fix the flickering of the screen with vt_vga, because the
sync signal is temporarily stopped and the video memory is cleared.

MFC after: 1 week