Qemu defines some common fwcfg items. We don't need to support all of
them. Only a subset needs to be present for fwcfg to work properly.
- signature The signature is used by the guest to check if qemu's fwcfg is available or not.
- id The id is used by the guest to check which features are supported by the fwcfg implementation of the hypervisor.
- file_dir The file dir reports all fwcfg items which don't have a fixed index. These are mostly user defined fwcfg items.