Merge citrix into master.

This was a somewhat difficult merge since there was a fair amount of
superficially divergent development on the two branches, especially in the
datapath.

This has been build-tested against XenServer 5.5.0 and XenServer 5.7.0
build 15122.  It has been booted and connected to XenCenter on 5.5.0.

The merge revealed a couple of outstanding bugs, which will be fixed on
citrix and then merged back into master.