Makefile: Align 'help' target output

The 'help' target is displayed unaligned. Add a print-help
function and use it. Now if someone want to change the
indentation, there is a single place to modify.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>