Move libvirt install + setup to functions-libvirt

Moves installation and setup of libvirt to a common functions-libvirt,
which can be used by other drivers in the future that may require
cross-distro libvirt installation and config but are not using
VIRT_DRIVER=libvirt (ie, Ironic).

Change-Id: I4a9255c8b4bacd5acfde9b8061c9e537aeea592c
Merge "Move libvirt install + setup to functions-libvirt"
Merge branch 'master' of https://github.com/openstack-dev/devstack