Fix install location for Python bindings

Fix install location for Python bindings

Commit 66137344feb488ea87b0d92f3c03844d9a7a7786 changed the Python detection
mechanism in configure to use AM_PATH_PYTHON. This results in a changed
install location for the Python bindings, at least on Fedora 12 64bit systems.

Before this commit libvirt.py and libvirtmod.so were installed to

  /usr/lib64/python2.6/site-packages

After this commit they are installed to

  /usr/lib/python2.6/site-packages

Mixed Python packages (containing *.py and *.so files) should be installed to
the pyexecdir directory detected by AM_PATH_PYTHON.

This restores the install location from before the AM_PATH_PYTHON commit.

* configure.in: remove unnecessary pythondir export
* python/Makefile.am: switch from pythondir to pyexecdir