vzdump: mailto: use email-or-username-list format

vzdump: mailto: use email-or-username-list format

because it is a more complete pattern. Also, 'mailto' was a '-list' format in
PVE 6.2 and earlier, so this also fixes whitespace-related backwards
compatibility. In particular, this fixes creating a backup job in the GUI
without setting an address, which passes along ''.

For example,
> vzdump 153 --mailto " ,,,admin@proxmox.com;;; developer@proxmox.com , ; "
was valid and worked in PVE 6.2.

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