torture: Make kvm-remote.sh give up on unresponsive system

Currently, a system that stops responding at the wrong time will hang
kvm-remote.sh.  This can happen when the system in question is forced
offline for maintenance, and there is currently no way for the user
to kick this script into moving ahead.  This commit therefore causes
kvm-remote.sh to wait at most 15 minutes for a non-responsive system,
that is, a system for which ssh gives an exit code of 255.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>