xenapi: Pass string arguments to popen

The current BitTorrent code was passing integer arguments to `make_subprocess`
which would in turn pass those to `popen`. This would cause a failure because
the semantics of the underlying `execve` function is to only accept a list of
strings.

The solution is to cast the arguments to a string before passing them into
`make_subrpocess`.

Fixes bug 1201594

Change-Id: I8372795ca032420641c449cb07bb47f9252ff4ef
Merge "xenapi: Pass string arguments to popen"