docs: mention vagrant-libvirt in apps.html

docs: mention vagrant-libvirt in apps.html

Doc patch for apps.html as per: http://libvirt.org/apps.html#add

Disclaimer: I've contributed patches to the project that this commit
adds.

Vagrant-Libvirt is an excellent way to use vagrant with libvirt. This
way you can benefit from the vagrant features, while not loosing access
to the familiar (and useful) tools such as virsh and virt-manager.

Development currently at:

https://github.com/pradels/vagrant-libvirt/

although recent contributors include:

https://github.com/sciurus/vagrant-libvirt/

and:

https://github.com/purpleidea/vagrant-libvirt/

see git log for more details.

Signed-off-by: Eric Blake <eblake@redhat.com>