snapshot: add two more filter sets to API

snapshot: add two more filter sets to API

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.