setup: Use user requested python binary
setup: Use user requested python binary

When setup.py is kicked off with a python interpreter other than the
system 'python', (e.g. python2.7 setup.py build) the build process would
switch to 'python' and not use python2.7 as requested by the user. We
should always respect the user requested python interpreter and use it.

libvirt/libvirt-python,libvirt/libvirt-python,libvirt/libvirt-python