x86/msr: Add dedicated macros to handle MSRs that are 64-bit only

Add a separate macro for handling 64-bit only MSRs to minimize churn and
copy+paste in a future commit that will add support for read-only bits.

Signed-off-by: Sean Christopherson <seanjc@google.com>
[Rebase on top of the fix for MSR_IA32_MISC_ENABLE, do not go overboard
 with macros. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>