xci: install: Move all pip installations to virtualenv

xci: install: Move all pip installations to virtualenv

Now that all pip installations are managed in the same way we can
move them inside a global virtual environment to further isolate the
XCI artifacts from the rest of the system. Moreover, we further simplify
the initial package installation to install everything at once instead
of calling the package manager for every single package that we need.

Change-Id: I6a170d2439fae8b0653f3141e0e8bb8ead67657e
Signed-off-by: Markos Chandras <mchandras@suse.de>