BugLink: http://bugs.launchpad.net/bugs/1616677

Use the virt_xx barriers that have been defined for use in virtual machines.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(back ported from commit dcd0eeca)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

 Conflicts:
	drivers/hv/ring_buffer.c
Acked-by: Brad Figg <brad.figg@canonical.com>
Acked-by: Kamal Mostafa <kamal@canonical.com>