virtio-console: Keep chardev open for other users after hot-unplug

virtio-console: Keep chardev open for other users after hot-unplug

After a hot-unplug operation, the previous behaviour was to close the
chardev.  That meant the chardev couldn't be re-used.  Also, since
chardev hot-plug isn't possible so far, this means virtio-console
hot-plug isn't feasible as well.

With this change, the chardev is kept around.  A new virtio-console
channel can then be hot-plugged with the same chardev and things will
continue to work.

Signed-off-by: Amit Shah <amit.shah@redhat.com>