KVM: VMX: Use proper type-safe functions for vCPU => LBRs helpers

Turn vcpu_to_lbr_desc() and vcpu_to_lbr_records() into functions in order
to provide type safety, to document exactly what they return, and to
allow consuming the helpers in vmx.h.  Move the definitions as necessary
(the macros "reference" to_vmx() before its definition).

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220727233424.2968356-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>