virt-admin: Introduce srv-clients-list command

virt-admin: Introduce srv-clients-list command

Wire-up the public client listing API. Along with this change, a private time
simple conversion method to interpret client's timestamp obtained from server
has been added as well. Format used to for time output is as follows:
YYYY-mm-DD HH:MM:SS+ZZZZ.

Although libvirt exposes methods time-related methods through virtime.h
internally, it utilizes millisecond precision which we don't need in this case,
especially when connection timestamps use precision to seconds only.
This is just a convenience int to string conversion method.

To reflect the new API, man page has been adjusted accordingly.

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