snapshot: Break out virDomainSnapshotObjList into its own file

snapshot: Break out virDomainSnapshotObjList into its own file

snapshot_conf.h was mixing three separate types: the snapshot
definition, the snapshot object, and the snapshot object list.
Separate out the snapshot object list code into its own file, and
update includes for affected clients.

This is just code motion, but done in preparation of sharing a lot of
the object list code with checkpoints.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>