ui/console: update the head from unused QemuConsole

When recycling unused QemuConsole, we should still set the associated
head number for correct information and lookups.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230830093843.3531473-25-marcandre.lureau@redhat.com>