Add vagrant libvert support in our multi-machine setup

[zeeshanlakhani]

Background

While VirtualBox (VB) is easiest to setup and is free, KVM/QEMU tends to have better performance and resource utilization (VB is notoriously bad about the latter); and,it's also free. Additionally, there are some bugs in VB around hardware performance counters (helpful for rr usage) that could be resolved with the addition of another virtualization provider.

Possible Solution

Add a configuration to our multi-machine Vagrantfile setup to use the vagrant-libvert in addition to VB (with a different config definition/name). Helpful instructions can be found in this reference.