virtio: console: Don't call hvc_remove() on unplugging console ports

hvc_remove() has some bug which freezes other active hvc ports when one
port is removed.

So disable calling of hvc_remove() which deregisters a port with the
hvc_console.

If the hvc_console code calls into our get_chars() routine as a result
of a poll operation, we will return -EPIPE and the hvc_console code will
then do the necessary cleanup.

This call will be restored when the bug in hvc_remove() is found and
fixed.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>