xen/PMU: Describe vendor-specific PMU registers

AMD and Intel PMU register initialization and helpers that determine
whether a register belongs to PMU.

This and some of subsequent PMU emulation code is somewhat similar to
Xen's PMU implementation.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>