Create aliases for named virtio-console ports.

Make virtio_console(4) create /dev/vtcon/<port_name> alias pointing
to /dev/ttyVx.y upon receiving PORT_NAME (id = 7) event over the control
queue.

Approved by: trasz
MFC after: 1 month
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D7182