bhyve: Add raw tcp to uart backend

This feature is required by OpenStack Nova that needs a serial output
through tcp socket. When enable this feature, a tcp server will be
started and wait for connection on specified port under capsicum's protection.
We only accept one connection at the same time. Other connection try to
connect will fail.

Reviewed by: corvink, markj
MFC after: 2 months
Differential Revision: https://reviews.freebsd.org/D45120