VMX: flush cache when vmentry back to UC guest

VMX: flush cache when vmentry back to UC guest

This patch flush cache when vmentry back to UC guest, to prevent
cache polluted by hypervisor access guest memory during UC mode.

The elegant way to do this is, simply add wbinvd just before vmentry.
However, currently wbinvd before vmentry will mysteriously trigger
lapic timer interrupt storm, hung booting stage for 10s ~ 60s. We still
didn't dig out the root cause of interrupt storm, so currently this
patch add flag indicating hypervisor access UC guest memory to prevent
interrupt storm -- though it still leaves aspects un-addressed, i.e.
speculative reads, and multi-vCPU issues, etc.

Whenever the interrupt storm got root caused and fixed, the protection
flag can be removed -- that would be final clean and elegant approach
dealing with cache flushing before vmentry.

This is CVE-2013-2212 / XSA-60.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Liu Jinsong <jinsong.liu@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Jun Nakajima <jun.nakajima@intel.com>