python: use PyBytes/PyUnicode instead of PyString

python: use PyBytes/PyUnicode instead of PyString

In Python2 PyBytes is the same as PyString, but in Python3 PyString is
gone and 'str' is really PyUnicode in C-API.
When handling arbitrary data, use PyBytes - which is the right thing to
do in Python3, and pose no API change in Python2. When handling
xenstore paths and transaction ids, which have well defined format, use
PyUnicode - to ease API usage - no need to prefix all xenstore paths
with 'b' when migrating scripts to Python3.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>