makedeb: Force Python setuptools to use Debian layout

By default, setuptools on Debian installs to /usr/local/lib
instead of /usr/lib - see https://wiki.debian.org/Python/Packaging:

   Python interpreter in Debian (since Python 2.6) is modified to use
   /usr/lib/pythonX.Y/dist-packages instead of site-packages. This
   way we can avoid conflicts with manually installed interpreter. It
   will also use /usr/local as --prefix by default (everyone who
   had to uninstall something ez_install installed (and overwrote
   our files), knows how important this change is), you can pass
   --install-layout=deb to setup.py's install to set all the Debian's
   defaults (f.e. --prefix=/usr).

This change writes a setup.cfg file to the root of the source package,
overriding the default.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
linux-guest-loader: Add build requirement on xcp-python-libs

linux-guest-loader does not require xcp-python-libs to be installed in
order to build it, but it does depend on it at run time.   xcp-python-libs
will still be built because it is a dependency of the 'all 'target, but
some other packages buildrequire linux-guest-loader, and since there is
no build dependency between linux-guest-loader and xcp-python-libs,
xcp-python-libs might not have been built at the time linux-guest-loader
is installed, so the installation will fail and stop the build.

Signed-off-by: Euan Harris <euan.harris@citrix.com>