the change introduced by commit 31c53c2e
do not allow anymore to have more than one monitor.

The purpose of this patch is to add an error message
when such a condition is identified, eg:
===
lxc-monitor: bind : Address already in use
===
Signed-off-by: Michel Normand <normand@fr.ibm.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>