mirage xen pcap example

[David Scott <scott.dj@xxxxxxxxx>]

Hi,

I did a bit of refactoring to the pcap stuff. It now consists of the
following pieces:

1. The master branch of mirage-net

https://github.com/mirage/mirage-net

has an extended "promiscuous hook"

2. The master branch of ocaml-pcap

https://github.com/mirage/ocaml-pcap

has a library "pcap.mirage"

There's an example showing how to turn it on in my branch of mirage-www:

https://github.com/djs55/mirage-www/tree/pcap

It will start appending packets to its first disk (disk 0 =/= 51712 =/= 'xvda')

I added "make run" which invokes "mir-run" and "xl", but it would also
be easy with xapi -- just make sure there's a disk for it to write to.

Cheers,
Dave