Update the -help output and documentation so that it recommends
'help' rather than '?' for the various "list valid values for this
option" cases. '?' is deprecated (as it can fail confusingly if
not quoted), so it's better to steer users towards 'help'. ('?'
still works, for backwards compatibility.)

This is the -help option part of the change otherwise done in
commit c8057f95, since we are now past release 1.2 and free to
change our help text without worrying about breaking libvirt.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>