list: Expose virStoragePoolListAllVolumes to Python binding

The implementation is done manually as the generator does not support
wrapping lists of C pointers into Python objects.

python/libvirt-override-api.xml: Document

python/libvirt-override-virStoragePool.py:
  * New file, includes implementation of listAllVolumes.

python/libvirt-override.c: Implementation for the wrapper.

libvirt-override-virStoragePool.py

@@ -0,0 +1,11 @@
+    def listAllVolumes(self, flags):
+        """List all storage volumes and returns a list of storage volume objects"""
+        ret = libvirtmod.virStoragePoolListAllVolumes(self._o, flags)
+        if ret is None:
+            raise libvirtError("virStoragePoolListAllVolumes() failed", conn=self)
+
+        retlist = list()
+        for volptr in ret:
+            retlist.append(virStorageVol(self, _obj=volptr))
+
+        return retlist