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

snapshot: s/parent/parent_name/ 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, and rewrite VIR_CLASS_NEW to call the new macro
with the hard-coded name 'parent', so that we could make
virDomainMomentDef use a custom name for its base class, 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, when we can no longer use 'parent' for a
different purpose than the base class.

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