The '-display' help information is not very correct. This patch sort
it a little.
Also, in its help information, reveals what implicit display option
will be chosen if no definition.

Signed-off-by: Robert Ho <robert.hu@intel.com>
Message-Id: <1469528231-26206-1-git-send-email-robert.hu@intel.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>