Wraps the conversion from 'char *name' to virDomainSnapshotPtr in
a reusable manner.

* src/conf/virdomainlist.h (virDomainListSnapshots): New declaration.
* src/conf/virdomainlist.c (virDomainListSnapshots): Implement it.
* src/libvirt_private.syms (virdomainlist.h): Export it.