virtio: console: don't assume a single console port.

Keep a list of all ports being used as a console, and provide a lock
and a lookup function.  The hvc callbacks only give us a vterm number,
so we need to map this.

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