libvirt: Configuration element for sVirt support

This configuration is need to allow users to enable sVirt support,
which is by default disabled for lxc.

In addition to a dynamic configuration, the users will be able to
provide a base label

Partially implements: blueprint libvirt-enable-lxc-svirt
Change-Id: Iacb6fcfb66379fbae900e8e9909377bf5ce18409
Merge "libvirt: Configuration element for sVirt support"