Define a decorator for libvirt handlers, those can be used only when the
connection to the libvirt is active

With the permission of Romain Degez, CTO of Smartjog.

Signed-off-by: Sébastien Luttringer <sebastien.luttringer@smartjog.com>

command not found/command doesn't have executable permission

Tunnel endpoints are now automaticaly closed by default.

Save loaded plugins list on every plugin install/remove
Load plugins list on startup