KVM: Add constant to represent KVM MSRs enabled bit in guest/host interface

This patch is simple, put in a different commit so it can be more easily
shared between guest and hypervisor. It just defines a named constant
to indicate the enable bit for KVM-specific MSRs.

Signed-off-by: Glauber Costa <glommer@redhat.com>
Acked-by: Rik van Riel <riel@redhat.com>
Tested-by: Eric B Munson <emunson@mgebm.net>
CC: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>