Add public API definition for data stream handling

Add public API definition for data stream handling

* include/libvirt/libvirt.h.in: Public API contract for
  virStreamPtr object
* src/libvirt_public.syms: Export data stream APIs
* src/libvirt_private.syms: Export internal helper APIs
* src/libvirt.c: Data stream API driver dispatch
* src/datatypes.h, src/datatypes.c: Internal helpers for virStreamPtr
  object
* src/driver.h: Define internal driver API for streams
* .x-sc_avoid_write: Ignore src/libvirt.c because it trips
  up on comments including write()
* python/Makefile.am: Add libvirt-override-virStream.py
* python/generator.py: Add rules for virStreamPtr class
* python/typewrappers.h, python/typewrappers.c: Wrapper
  for virStreamPtr
* docs/libvirt-api.xml, docs/libvirt-refs.xml: Regenerate
  with new APIs