xend, pci passthru: Use Dstate transition to do FLR for integrated

xend, pci passthru: Use Dstate transition to do FLR for integrated
devices without proper FLR capability.

Currently integrated devices without proper FLR capability are not
assignable. This is not nice since actually they may be assignable,
but due to old BIOS the proper FLR capability is not exposed. We can
use the previously-used Dstate transition method to do FLR for them.
This works well and looks few issues are reported AFAIK. Maybe we can
make a black list for the (rare) integrated devices that don't work
with this method.

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>