Track resources and enforce cpu/memory/interface limits for non-root users.

Track resources and enforce cpu/memory/interface limits for non-root users.

The limits are currently hard-coded and undocumented (4 CPUs/VMs, 2G
memory, 8 interfaces) but will be configurable in an upcoming diff.
These limits are tracked in total usage; for example, a user will be
able to run up to 4 VMs with 512M of memory or a single VM with 2G.

OK ccardenas@ mlarkin@