kvm tools: Add own barrier() definition

kvm tools: Add own barrier() definition

Otherwise I'm getting the following compile problem on my Fedora
machine. The helper is rather taken from linux kernel.

 | [cyrill@moon kvm]$ make tags
 | x86/include/kvm/barrier.h:11:25: fatal error: asm/barrier.h: No such file or directory compilation terminated.

Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pekka Enberg <penberg@kernel.org>