Error early if the box VM would overcommit the host memory

[kageurufu]

I was testing how boxed works on low memory systems, and it will readily overcommit memory and get quickly reaped by the OoM killer.

It would be useful to add a quick check that the requested box memory is larger than the free/available memory instead of just crashing a host.

Came here from rockstor/rockstor-installer#73 btw