Add telnet connection retry
Add telnet connection retry

I've seen on a few how vrnetlab fails to start because it tries to
connect to the Qemu monitor before qemu has properly started, so instead
of defining some fixed sleep we do a more dynamic retry.

plajjan/vrnetlab,mzagozen/vrnetlab,mzagozen/vrnetlab,bdreisbach/vrnetlab,plajjan/vrnetlab,bdreisbach/vrnetlab