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

The tag was not removed from TagDB when the libvirt connection was lost.

With the permission of Romain Degez, CTO of Smartjog.

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

Tunnel endpoints are now automaticaly closed by default.

Added the type in the method signature in cc-common.

Moved to cc-common

Was using a closure for tag function thus storage variable was always
referencing the last value of the iteration.

hypervisor/__init__.py ->
    hypervisor/__init__.py (Handler)
    hypervisor/kvm.py (Hypervisor -> KVM)
    hypervisor/lib.py (Storage*/Volume)