remote: extract logic for probing for modular daemons

remote: extract logic for probing for modular daemons

When virtproxyd gets a NULL URI, it needs to implement probing logic
similar to that found in virConnectOpen. The latter can't be used
directly since it relied on directly calling into the internal drivers
in libvirtd. virtproxyd approximates this behaviour by looking to see
what modular daemon sockets exist, or what daemon binaries are installed.

This same logic is also going to be needed when the regular libvirt
remote client switches to prefer modular daemons by default, as we
don't want to continue spawning libvirtd going forward.

Tested-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>