Add support for libvirt provider for development machine
Add support for libvirt provider for development machine

- Uses new `availabe_cpus` Ruby function to ensure all CPUs are used.
- Although `config.vm.synced_folder` defaults to the exact settings in this
  commit, if NFS support is not present on the host, Vagrant will fall back
  to unidirectional `rsync` syncing. It's better to fail and make the user
  setup NFS support than leave w/o bidirectional syncing support.

heartsucker/securedrop,ageis/securedrop,micahflee/securedrop,micahflee/securedrop,ehartsuyker/securedrop,garrettr/securedrop,conorsch/securedrop,ehartsuyker/securedrop,micahflee/securedrop,conorsch/securedrop,conorsch/securedrop,heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,ageis/securedrop,ehartsuyker/securedrop,ageis/securedrop,ehartsuyker/securedrop,garrettr/securedrop,ehartsuyker/securedrop,garrettr/securedrop,ageis/securedrop,micahflee/securedrop,garrettr/securedrop,conorsch/securedrop,heartsucker/securedrop,heartsucker/securedrop