vzdump: verify parameters: properly verify notes-template

vzdump: verify parameters: properly verify notes-template

instead of just checking for a newline, do a full check already.

Also do the check at the beginning of generate_notes() for consistency
and remove the check after expansion to avoid failing late for things
like '{{cl{{node}}er}}' (which can even expand to a valid variable
making the error even more confusing).

Co-developed-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>