shared.scripts.virtio_console_guest: Adds new loopback mode

This patch adds _reconnect_none_mode version of the loopback thread.
It works as ordinary none_mode, but additionally disconnects in case
the port is unreadable and reconnects when possible.

It was necessarily to make virt global variable to be able to access
it in this thread.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>