Snapshot filtering based on types is useful enough to add
back-compat support into virsh. It is also rather easy - all
versions of libvirt that don't understand the new filter flags
already gave us sufficient information in a single XML field
to reconstruct all the information we need (that is, it isn't
until libvirt 1.0.1 that we have more interesting types of
snapshots, such as offline external).
* tools/virsh-snapshot.c (vshSnapshotFilter): New function.
(vshSnapshotListCollect): Add fallback support.