KVM: SVM: Use svm_msrpm_offset in nested_svm_exit_handled_msr

There is a generic function now to calculate msrpm offsets.
Use that function in nested_svm_exit_handled_msr() remove
the duplicate logic (which had a bug anyway).

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>