snapshot: use metaroot node to simplify management

snapshot: use metaroot node to simplify management

This idea was first suggested by Daniel Veillard here:
https://www.redhat.com/archives/libvir-list/2011-October/msg00353.html

Now that I am about to add more complexity to snapshot listing, it
makes sense to avoid code duplication and special casing for domain
listing (all snapshots) vs. snapshot listing (descendants); adding
a metaroot reduces the number of code lines by having the domain
listing turn into a descendant listing of the metaroot.

Note that this has one minor pessimization - if we are going to list
ALL snapshots without filtering, then virHashForeach is more efficient
than recursing through the child relationships; restoring that minor
optimization will occur in the next patch.

* src/conf/domain_conf.h (_virDomainSnapshotObj)
(_virDomainSnapshotObjList): Repurpose some fields.
(virDomainSnapshotDropParent): Drop unused parameter.
* src/conf/domain_conf.c (virDomainSnapshotObjListGetNames)
(virDomainSnapshotObjListCount): Simplify.
(virDomainSnapshotFindByName, virDomainSnapshotSetRelations)
(virDomainSnapshotDropParent): Match new field semantics.
* src/qemu/qemu_driver.c (qemuDomainSnapshotCreateXML)
(qemuDomainSnapshotReparentChildren, qemuDomainSnapshotDelete):
Adjust clients.