kvm: x86: use macros to compute bank MSRs

commit 81760dcc

 upstream.

Avoid open coded calculations for bank MSRs by using well-defined
macros that hide the index of higher bank MSRs.

No semantic changes.

Signed-off-by: Chen Yucong <slaoub@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>