Minimega: Delay in vm info

[aherna]

Describe your environment

1. minimega version
   all
2. Linux distro/version
3. Go compiler version
   all
4. VM types
   many

Describe the bug
Once there are many VMs there a significant delay (ms to seconds) when calling Info in /minimega/vm.go for the “qos” case. The upper bound for the delay is when there is a lot of other disk activity (multiple memory dumps, etc.)
it looks like the "bridges" file is being used to track bridges that minimega used while running. The purpose of the tracking appears to be for cleanup purposes (minimega/nuke.go – nukeBridges) to delete bridges that minimega added or used/pre-existed before minimega.

Expected behavior
Quicker response time