ui: backup: fill in some of the configured vzdump defaults

ui: backup: fill in some of the configured vzdump defaults

Do not fill in the default for compression, because the initial default for the
backend is to not compress, while the current default for the UI is zstd, which
is preferable.

The 'defaults' API call expects the user to have permissions on the storage,
because retention options are storage-dependent. Use a flag initialDefaults to
make sure storage-independent properties are only set once, so they are not
reset when a user changes the storage after editing them.

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