kvm: vmx: Rename function find_msr() to vmx_find_msr_index()

Rename function find_msr() to vmx_find_msr_index() in preparation for an
upcoming patch where we export it and use it in nested.c.

Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>