import: append % to X_IMPORT_PROGRESS=

import: append % to X_IMPORT_PROGRESS=

Since we parse it on the other side via parse_percent() which requires
that, otherwise we get an error:

[    8.133131] testsuite-13.sh[649]: + machinectl import-raw /tmp/container.raw container-raw
[    8.175035] machinectl[1143]: Enqueued transfer job 1. Press C-c to continue download in background.
[    8.182130] machinectl[1143]: Importing '/tmp/container.raw', saving as 'container-raw'.
[    8.182377] systemd-importd[1144]: Got invalid percent value '0', ignoring.
[    8.182451] machinectl[1143]: Imported 0%.
[    8.282669] systemd-importd[1144]: Got invalid percent value '40', ignoring.
[    8.282746] machinectl[1143]: Imported 40%.
[    8.366448] machinectl[1143]: Wrote 64.0M.
[    8.366519] machinectl[1143]: Operation completed successfully.
[    8.366617] machinectl[1143]: Exiting.