Allow setting alternative qrexec service to retrieve backup content. The
service API is slightly different than the default one: it will get only
list of files/directories to extract on its stdin, but not backup
location. The latter could be provided as a service argument, or using
other out-of-band mechanism.
This will be useful for paranoid backup restore mode, to take away
control over location/command from sandboxed qvm-backup-restore process.
QubesOS/qubes-issues#5310
db1d4b5d48