list: provide python bindings for snapshots

list: provide python bindings for snapshots

This adds support for the new virDomainListAllSnapshots (a domain
function) and virDomainSnapshotListAllChildren (a snapshot function)
to the libvirt-python bindings.  The implementation is done manually
as the generator does not support wrapping lists of C pointers into
python objects.

* python/libvirt-override.c (libvirt_virDomainListAllSnapshots)
(libvirt_virDomainSnapshotListAllChildren): New functions.
* python/libvirt-override-api.xml: Document them.
* python/libvirt-override-virDomain.py (listAllSnapshots): New
file.
* python/libvirt-override-virDomainSnapshot.py (listAllChildren):
Likewise.
* python/Makefile.am (CLASSES_EXTRA): Ship them.