python: Use hardcoded python path in libvirt.py

python: Use hardcoded python path in libvirt.py

This partially reverts (and fixes that part in a different way) commit
e4384459c93e3e786aa483c7f077d1d22148f689, which replaced
``/usr/bin/python'' with ``/usr/bin/env python'' in all examples or
scripts used during build to generate other files.

However, python bindings module is compiled and linked against a
specific python discovered or explicitly provided in configure phase.
Thus libvirt.py, which is generated and installed into the system,
should use the same python binary for which the module has been built.

The hunk in Makefile.am replaces $(srcdir) with $(PYTHON), which might
seem wrong but it is not. generator.py didn't use any of its command
line arguments so passing $(srcdir) to it was redundant.