admin: Introduce virAdmClient client-side object

admin: Introduce virAdmClient client-side object

Besides ID, the object also stores static data like connection transport and
connection timestamp, since once obtained a list of all clients connected to a
server, from user's perspective, it would be nice to know whether a given
client is remote or local only and when did it connect to the daemon.
Along with the object introduction, all necessary client-side methods necessary
to work with the object are added as well.

Signed-off-by: Erik Skultety <eskultet@redhat.com>