vzdump: actually honor schema defaults for performance

vzdump: actually honor schema defaults for performance

The 'performance' option itself defines no 'default' in the schema, so
what happened is that the defaults used by the backends (i.e. QEMU and
proxmox-backup-client) would be used. Luckily, they correspond to the
default values defined in the schema, i.e. in the 'backup-performance'
format. Make the code future-proof and use the actual defaults defined
in the schema instead of relying on that correspondence.

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