Now that domain listing is a thin wrapper around child listing,
it's easier to have a common entry point.  This restores the
hashForEach optimization lost in the previous patch when there
are no snapshots being filtered out of the entire list.

* src/conf/domain_conf.h (virDomainSnapshotObjListGetNames)
(virDomainSnapshotObjListNum): Add parameter.
(virDomainSnapshotObjListGetNamesFrom)
(virDomainSnapshotObjListNumFrom): Delete.
* src/libvirt_private.syms (domain_conf.h): Drop deleted functions.
* src/conf/domain_conf.c (virDomainSnapshotObjListGetNames):
Merge, and (re)add an optimization.
* src/qemu/qemu_driver.c (qemuDomainUndefineFlags)
(qemuDomainSnapshotListNames, qemuDomainSnapshotNum)
(qemuDomainSnapshotListChildrenNames)
(qemuDomainSnapshotNumChildren): Update callers.
* src/qemu/qemu_migration.c (qemuMigrationIsAllowed): Likewise.
* src/conf/virdomainlist.c (virDomainListPopulate): Likewise.