[HVM][SVM] Reintroduce ASIDs.

[HVM][SVM] Reintroduce ASIDs.

ASIDs partition the physical TLB for SVM.  In the current implementation
ASIDs are used to reduce the number of TLB flushes.  Each time the
guest's virtual address space changes (e.g. due to an INVLPG,
MOV-TO-{CR3, CR4} operation), instead of flushing the TLB, a new ASID is
assigned.  This reduces the number of TLB flushes to at most 1/#ASIDs
(currently 1/64).  The biggest advantage is that hot parts of the
hypervisor's code and data remain in the TLB.

From: Sebastian Biemueller <Sebastian.Biemueller@amd.com>
Signed-off-by: Keir Fraser <keir@xensource.com>