Implement pre-flight check for USB export

[eloquence]

When we export files to a USB drive, we'll need to prompt the user for the export passphrase the first time they use a Transfer Device in a given session. (It will be cached iff it has been successfully used at least once.) But we don't want to prompt the user for the passphrase unless we know the export drive can be successfully attached to the export VM in the first place, and is of the correct type.

For this reason it would be useful to have a "pre-flight check" that performs these verifications, lets the client know the result, and if the check succeeds, it can prompt the user for the passphrase. See this chart for the user experience workflow:

https://www.figma.com/proto/0q9J1nt8URwppCPQwX7cLfY0/export22?node-id=0%3A2&scaling=min-zoom&redirected=1

The two checks are:

• a USB device is attached to the export port
• the USB device appears to be an encrypted storage device