(gh-552) Add support for vagrant_libvirt

Currently vagrand backend support is limited to virtualbox,
vmware_workstation and fusion. Vagrant offers a non-official
libvirt backend through the vagrant-libvirt[1] plugin.
This commit aims to add support for vagrant_libvirt.

[1] https://github.com/pradels/vagrant-libvirt
Merge pull request #551 from Mylezeem/support_vagrant_libvirt

(GH-552) Hypervisor: Add support for vagrant_libvirt