vzdump: have property string helpers always return the result

vzdump: have property string helpers always return the result

Previously, the result would only be returned implicitly and if not
already parsed. While callers do not strictly need the return value,
future callers might mistakenly rely on it and even work by chance in
some scenarios, because of the implicit return. Make the code more
future proof by explicitly returning the result in all cases.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>