[stubdom] fix the crash of HVM live migration with intensive disk access

[stubdom] fix the crash of HVM live migration with intensive disk access

Intensive disk access, e.g. sum of big file, during HVM live migration would
cause guest error even file system crash. Guest dmesg said
"attempt to access beyond end of device
hda1: rw=0, want=10232032112, limit=10474317"

Current map cache used by qemu dma doesn't mark the page dirty, so that these
pages(probably holding DMA data struct) are not transferred in the last
iteration during live migration.

This patch fixes it, and also merges the qemu's original dirty bitmap used by
other devices such as vga.

Signed-Off-By: Zhai Edwin <edwin.zhai@intel.com>