nixos/virtualbox-image: increase disk to 50G
nixos/virtualbox-image: increase disk to 50G

100GB breaks cptofs but 50GB is fine and benchmarks shows it takes the same time as building the demo VBox VM with a 10GB disk

+ enabled VM sound output by default
+ set USB controller in USB2.0 mode
+ add manifest file in the OVA as it allows integrity checking on imports
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