Add option to use SLIRP NAT for external connections; enable pcap for vpnguin traffic

[zestrada]

A possible implementation for #499

Future versions of QEMU allow us to specify a network other than 10.0.2.0/24 so I've got that code in there but commented out.

This does add a top-level network configuration, with optional support for pcap capture. Example usage:

network:
  external:
    pcap: true

Definitely open to feedback on implementation, interface, etc...

[zestrada]

Updated to add support for VPN pcap as well, after merge of rehosting/vpnguin#24

plugins:
  vpn:
    pcap: true

When enabled, a vpn.pcap file will be generated in outdir