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

python/libvirt-override-virStream.py

@@ -0,0 +1,20 @@
+    def __del__(self):
+        try:
+            if self.cb:
+                libvirtmod.virStreamEventRemoveCallback(self._o)
+        except AttributeError:
+           pass
+
+        if self._o != None:
+            libvirtmod.virStreamFree(self._o)
+        self._o = None
+
+    def eventAddCallback(self, cb, opaque):
+        """ """
+        try:
+            self.cb = cb
+            self.opaque = opaque
+            ret = libvirtmod.virStreamEventAddCallback(self._o, self)
+            if ret == -1: raise libvirtError ('virStreamEventAddCallback() failed', conn=self._conn)
+        except AttributeError:
+            pass