KVM: x86: Refactor up kvm_{g,s}et_msr() to simplify callers

Refactor the top-level MSR accessors to take/return the index and value
directly instead of requiring the caller to dump them into a msr_data
struct.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>