kvm: Use -display none rather than -nographic

Newer versions of kvm (from 1.4) don't support -nographic with
-daemonize. When -display exists, use -display none instead.

This fixes Issue 389.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Thomas Thrainer <thomasth@google.com>