Add FAQ item about common puppet failures and the workaround.
Set the libvirt driver to use accelerated kvm.

This wasn't necessary in previous versions of vagrant-libvirt because
this option specified the uri, and the domain type defaulted to kvm. In
newer releases, you have to specify this for performance, otherwise it
will default to qemu software only virtualization. This depends on:

https://github.com/pradels/vagrant-libvirt/commit/67734f003273d3fadcca3893c994f691fee1b629

also available at:

https://github.com/purpleidea/vagrant-libvirt/commit/67734f003273d3fadcca3893c994f691fee1b629