snapshot: s/current/parent/ as prep for virObject

snapshot: s/current/parent/ as prep for virObject

VIR_CLASS_NEW insists that descendents of virObject have 'parent' as
the name of their inherited base class member at offset 0. While it
would be possible to write a new class-creation macro that takes the
actual field name 'current', and rewrite VIR_CLASS_NEW to call the new
macro with the hard-coded name 'parent', it seems less confusing if
all object code uses similar naming. Thus, this is a mechanical rename
in preparation of making virDomainSnapshotDef a descendent of
virObject.

Signed-off-by: Eric Blake <eblake@redhat.com>
Acked-by: Peter Krempa <pkrempa@redhat.com>