This is a pretty simple patch to use the read/write barriers defined in asm/system.h instead of using hardcoded versions in various places throughout Xen.

I've checked and using asm/system.h generates the same code on my system. I also tested xcs and it seems to work fine. I haven't tested blktap but again, it's generating the same code.

Right now, wmb() is defined as a NOP on any 386 architecture. Some Intel clones require a non-NOP wmb(). Using asm/system.h ensures we do the right thing.

It's against xen-unstable as of today.