Some classes require nova::compute::libvirt but there is no need to
put that class into dependencies because resource orders are defined
by deps or explicit definitions. Also, the base libvirt class is not
really required.
This change removes these dependencies, to avoid enforcing orders as
much as possible.
Change-Id: Icbb2fa209effd355cef36f35662bfdcf18d8b090
e192c9c789