shared.scripts.virtio_console_guest: Fix problem with async mode

The bacground process was likely to be killed by SIGIO while setting
port to async mode. This was due of bad workflow, where first the
port was set to async mode, than the the new SIGIO handler was set.

With this version when we set async mode, we fist add the SIGIO
handler and then set the port to async mode.

When we are setting the port back, the order is the opposite, so
first the port is set to sync mode and then we remove the SIGIO
handler.

Additionally the workaround with test_delete_guest_script() was
removed as it's fixed by this patch.

NOTE: You have to remove the previous /tmp/virtio_console_guest.py*
      in order to let virtio_console test to copy this updated one!

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>