KVM: Add Documentation/kvm/msr.txt

This patch adds a file that documents the usage of KVM-specific
MSRs.

Signed-off-by: Glauber Costa <glommer@redhat.com>
Reviewed-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>