virDomainSnapshotRedefinePrep: Don't do partial redefine

virDomainSnapshotRedefinePrep: Don't do partial redefine

'virDomainSnapshotRedefinePrep' does everything needed for a redefine
when the snapshot exists but not when we are defining metadata for a new
snapshot. This gives us weird semantics.

Extract the code for replacing the definition of an existing snapshot
into a new helper 'virDomainSnapshotReplaceDef' and refactor all
callers.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>