Remove #VMs <= 256 limit

[sriramy]

At some point the limit #VMs <= 256 may be addressed (not a limitation in my setups yet), but the limit is actually 100 since gnu screen have a max-limit of 100. Now, that can be configured ad build-time, but better to seek an alternative to screen, like a simple pipe or something

[uablrek]

IMO this should have very low prio, and only be implemented if a real need arises.

It hits hard on the entire network setup and several (convenient) assumptions. A test that really needs >256 nodes can most likely be implemented in an ovl, albeit a pretty obscure one with hooks and things.