qemu-xen: ignore console disconnect events for console/0

qemu-xen: ignore console disconnect events for console/0

The first console has a different location compared to other PV devices
(console, rather than device/console/0) and doesn't obey the xenstore
state protocol. We already special case the first console in con_init
and con_initialise, we should also do it in con_disconnect.

This patch should be applied to 4.1 too.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>