utils: Remove for-loop variable decl.

Breaks Python distutils which doesn't use C99:

utils.c: In function ‘guestfs___copy_string_list’:
utils.c:79:7: error: ‘for’ loop initial declarations are only allowed in C99 mode
       for (size_t j = 0; j < i; ++j)
       ^
utils.c:79:7: note: use option -std=c99 or -std=gnu99 to compile your code
utils: Move guestfs___validate_guid out of utils.c.

guestfs___validate_guid was a new function added to utils.c in
commit beef77403cd9d634b6ff6daa9f33d292e2d320a7.

However utils.c should not include <guestfs-internal.h> since the
other functions in this file can be used by all front-end code, not
just libguestfs.so.

This function is only needed in libguestfs.so, so move it to another
source file, and remove include of <guestfs-internal.h> from utils.c.

Also: use 'size_t' for counting, not 'int'.

This fixes commit beef77403cd9d634b6ff6daa9f33d292e2d320a7.
python: Add a Python setup.py script.

This is not used by libguestfs right now, but it does allow you to
build a python "distribution" of libguestfs, like this:

  make -C python sdist

The distribution tarball will be in python/dist/

You can copy the distribution tarball somewhere, unpack it, and use
regular 'python setup.py' commands such as:

  python setup.py build
  python setup.py install [--prefix=...]
  python setup.py --name

In future we hope to be able to upload the distribution tarball to
PyPi, but licensing issues prevent this at present.