VMX: fix cr0.cd handling

VMX: fix cr0.cd handling

This patch solves XSA-60 security hole:
1. For guest w/o VT-d, and for guest with VT-d but snooped, Xen need
do nothing, since hardware snoop mechanism has ensured cache coherency.

2. For guest with VT-d but non-snooped, cache coherency can not be
guaranteed by h/w snoop, therefore it need emulate UC type to guest:
2.1). if it works w/ Intel EPT, set guest IA32_PAT fields as UC so that
guest memory type are all UC.
2.2). if it works w/ shadow, drop all shadows so that any new ones would
be created on demand w/ UC.

This patch also fix a bug of shadow cr0.cd setting. Current shadow has a
small window between cache flush and TLB invalidation, resulting in possilbe
cache pollution. This patch pause vcpus so that no vcpus context involved
into the window.

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

Signed-off-by: Liu Jinsong <jinsong.liu@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jun Nakajima <jun.nakajima@intel.com>
Acked-by: Keir Fraser <keir@xen.org>