Add a pair of services:
1. qubes.RegisterBackupLocation - called by dom0, registers what backup
location (including both file and command options) can be accessed.
Registered location gets an ID returned to the caller. The location (and
its ID) is valid as long as the service call remains open.
2. qubes.RestoreById - called by restoring DispVM to retrieve the backup
content. The service expects location ID as an argument, and then list
of files/directories (separated with spaces) on the first line of stdin.
This is very similar to qubes.Restore service, with exception for the
archive location control.
QubesOS/qubes-issues#5310
