snapshot: Tweaks to bulk dumpxml/import internals

snapshot: Tweaks to bulk dumpxml/import internals

Change the return value of virDomainSnapshotObjListParse() to return
the number of snapshots imported, and allow a return of 0 (the
original proposal of adding a flag to virDomainSnapshotCreateXML
required returning an arbitrary non-NULL snapshot, but that idea was
abandoned; and by returning a count, we are no longer constrained to a
non-empty list).

Document which flags are supported (namely, just SECURE) in
virDomainSnapshotObjListFormat().

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>