remote: fix handling of systemd activation wrt socket ordering

remote: fix handling of systemd activation wrt socket ordering

The current libvirtd code for systemd socket activation assumes socket
FDs are passed in the order unix-rw, unix-ro, unix-admin.  There is in
fact no ordering guarantee made by systemd. Applications are expected
to check the address or name associated with each FD to figure out its
identity.

This rewrites libvirtd to make use of the new systemd activation APIs
to make it robust wrt socket ordering changes.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>