Port forwarding support for QEMU

This adds a "-f" command line option to "capstan run" for specifying
port forwarding rules. 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 -f 9090:8080

makes the guest 8080 port available at localhost:9090.

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