staging: unisys: remove MEMORYBARRIER AND VolatileBarrier from commontypes

Remove the MEMORYBARRIER and VolatileBarrier() defines from commontypes.h and
convert the spots that used this to mb(). Add comments to each use to indicate
that the barrier is used to ensure channel synchronization between guests.

Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>