Refactor get_msr_feature() to take the index and data pointer as distinct
parameters in anticipation of eliminating "struct kvm_msr_entry" usage
further up the primary callchain.

No functional change intended.

Link: https://lore.kernel.org/r/20240802181935.292540-5-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>