vbox: Port forwarding

This adds port forwarding support to vbox via the "-f" command line
option that is already supported by QEMU.

The syntax of a port forward rule is:

  <host-port>:<guest-port>

For example, running Don's HTTP server example with:

  [penberg@localhost http-server]$ capstan run -p vbox -f 9090:8080

makes the guest 8080 port available at localhost:9090.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>