list: add virDomainListAllSnapshots API

list: add virDomainListAllSnapshots API

There was an inherent race between virDomainSnapshotNum() and
virDomainSnapshotListNames(), where an additional snapshot could
be created in the meantime, or where a snapshot could be deleted
before converting the name back to a virDomainSnapshotPtr.  It
was also an awkward name: the function operates on domains, not
domain snapshots.  virDomainSnapshotListChildrenNames() suffered
from the same inherent race, although its naming was nicer.

This patch makes things nicer by grabbing a snapshot list
atomically, in the format most useful to the user.

* include/libvirt/libvirt.h.in (virDomainListAllSnapshots)
(virDomainSnapshotListAllChildren): New declarations.
* src/libvirt.c (virDomainSnapshotListNames)
(virDomainSnapshotListChildrenNames): Add cross-references.
(virDomainListAllSnapshots, virDomainSnapshotListAllChildren):
New functions.
* src/libvirt_public.syms (LIBVIRT_0.9.13): Export them.
* src/driver.h (virDrvDomainListAllSnapshots)
(virDrvDomainSnapshotListAllChildren): New callbacks.
* python/generator.py (skip_function): Prepare for later
hand-written versions.