qemu: Bridged networking support

This patch adds bridged networking support to Capstan. You can enable it
with the "-n" command line option:

  $ capstan run -n bridge

which will use 'virbr0' device by default. You can change it with the
'-b' command line option.

Capstan will still default to port forwarding but we definitely should
switch to bridging once we support it on other hypervisors as well.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>