This patch adds a new dom0 plugin which supports downloading images via
BitTorrent. Torrent metadata files are assumed to be served from a
webserver which is specified by the `torrent_base_url` config.
Under the hood, the dom0 plugins calls out to rasterbar's libtorrent via
Python bindings in order perform the initial download as well as the
seeding thereafter.
Implements BP xenserver-bittorrent-images
Change-Id: I824720a6e3a37317080a22cd7405d2a88172c3ef
216a83da47