Hello!

I would like to suggest a little extension in the manual.

I noticed that the package variable for virtlogd service is not
documented.  It is possible to define the package as (libvirt libvirt)
for said service.  While it is closely related to libvirt-service-type,
the package variable may still be independently modified.

This only became clear after using inferiors in libvirt-service-type for
some time now.  New Qemu packages would still be pulled in after
reconfiguring with new checkouts.  Unfortunately Qemu's packages take a
lot of space, which is why I've pinned it to a fixed version.

Thus I think this should be included into the manual.