KVM: VMX: Enable Virtual Processor Identification (VPID)

To allow TLB entries to be retained across VM entry and VM exit, the VMM
can now identify distinct address spaces through a new virtual-processor ID
(VPID) field of the VMCS.

[avi: drop vpid_sync_all()]
[avi: add "cc" to asm constraints]

Signed-off-by: Sheng Yang <sheng.yang@intel.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>