As we enable more modes of snapshot creation, it becomes more important
to be able to quickly filter based on snapshot properties.  This patch
introduces new filter flags; subsequent patches will introduce virsh
back-compat filtering, as well as actual libvirt filtering.

* include/libvirt/libvirt.h.in (virDomainSnapshotListFlags): Add
five new flags in two new groups.
* src/libvirt.c (virDomainSnapshotNum, virDomainSnapshotListNames)
(virDomainListAllSnapshots, virDomainSnapshotNumChildren)
(virDomainSnapshotListChildrenNames)
(virDomainSnapshotListAllChildren): Document them.
* src/conf/snapshot_conf.h (VIR_DOMAIN_SNAPSHOT_FILTERS_STATUS)
(VIR_DOMAIN_SNAPSHOT_FILTERS_LOCATION): Add new convenience filter
collection macros.
* tools/virsh-snapshot.c (cmdSnapshotList): Add 5 new flags.
* tools/virsh.pod (snapshot-list): Document them.