x86_emulate: Emulate CLFLUSH instruction

x86_emulate: Emulate CLFLUSH instruction

We recently found that FreeBSD 8.0 guest failed to install and boot on
Xen. The reason was that FreeBSD detected clflush feature and invoked
this instruction to flush MMIO space. This caused a page fault; but
x86_emulate.c failed to emulate this instruction (not supported). As a
result, a page fault was detected inside FreeBSD. A similar issue was
reported earlier.

http://lists.xensource.com/archives/html/xen-devel/2010-03/msg00362.html

From: Wei Huang <wei.huang2@amd.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>