Don't check for xend port when using xl

If the toolstack is set to "xl" we shouldn't ping xend for liveness
before attempting a live migration.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>