When enabling log dirty mode, it sets all guest's memory to readonly.

When enabling log dirty mode, it sets all guest's memory to readonly.
And in HAP enabled domain, it modifies all EPT entries to clear write bit
to make sure it is readonly. This will cause problem if VT-d shares page
table with EPT: the device may issue a DMA write request, then VT-d engine
tells it the target memory is readonly and result in VT-d fault.

Currnetly, there are two places will enable log dirty mode: migration and vram
tracking. Migration with device assigned is not allowed, so it is ok. For vram,
it doesn't need to set all memory to readonly. Only track the vram range is enough.

Signed-off-by: Yang Zhang <yang.z.zhang@Intel.com>
Acked-by: Tim Deegan <tim@xen.org>