bug#53921: The libvirt service does not contain a way to turn-on remote listening

[Katherine Cox-Buday]

The shepherd libvirt service contains ways to configure "listening mode" (i.e. 
listening over TCP) capabilities, but contains no way to actually turn this 
feature on, despite referencing[1] an unimplemented `listen` option:

> You must set listen for this to have any effect. 

>From libvirt's documentation[2]:

> Note: it is also necessary to start the server in listening mode by
> running it with --listen or adding a LIBVIRTD_ARGS="--listen" line to
> /etc/sysconfig/libvirtd.

Our service definition requires the ability to specify that shepherd should 
start the service with the `--listen` flag.

[1] - https://guix.gnu.org/manual/en/guix.html#index-listen_002dtcp_003f
[2] - https://libvirt.org/remote.html#Remote_libvirtd_configuration

-- 
Katherine