Enable guest OS to program D0-D3hot states of an assigned device

Enable guest OS to program D0-D3hot states of an assigned device

This patch enables guest OS to program D0-D3hot states of assigned
device.

This patch is revised version based on the review comments.

- Use LIST_FOREACH(). Don't use lh_first, le_next directly.
- Use pci_{read, write}_block instead of "switch(len) case [124]
  pci_{read, write}_{byte, word, long}():".

Signed-off-by: Yuji Shimada <shimada-yxb@necst.nec.co.jp>