Explicitly close libvirt connections at exist

When a process (such as Vdsm, or `vdsm-tool dummybr`) exists while
holding an open libvirt connection, libvirt logs an error

    : error : virNetSocketReadWire:1184 : End of file while reading data: Input/output error

This patch close all libvirt connections at process exit.

Change-Id: I2b15f3ab017828a63960ba9311fa2bf6bfab4729
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17192
Reviewed-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-by: mooli tayer <mtayer@redhat.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>