Add sd-export-config.json in sd-export VMs

[emkll]

Towards #281

This will allow sd-export to know what the dedicated USB device is for the export VM to facilitate preflight checks. This is required for the securedrop-export script to be able to know what the usb device configuration is. This is required for the USB preflight check, to check if a device is connected or not.

Test plan

• make sd-export completes without error
• make test completes without error
• observe the configuration is populated in sd-export-usb (example below)

user@sd-export-usb:~$ cat /etc/sd-export-config.json 
{
  "pci_bus_id": "2",
  "usb_device": "4"
}

[conorsch]

• make sd-export completes without error
• make test completes without error
• observe the configuration is populated in sd-export-usb

Works as advertised!