FortiOS VM and reserved disk space for logs

In earlier patches of FortiOS 6.0 VM, the virtual disk is shared by logging and the WAN Optimization and Cache features, regardless what is enabled or required. When upgrading from FortiOS 5.6 to 6.0.0 through 6.0.2, the VM's disk was partitioned and formatted, causing logs to be lost. A change in FortiOS 6.0.3 and later aligns the disk usage of VM models with the disk usage of FortiGate D-series hardware devices, allowing administrators to dedicate disks as required and keep a single partition. Upgrades from FortiOS 5.6 to 6.0.3 and later will keep the disk intact, without losing logs.