backup: fix deadlock on error while receiving backup from a VM

When qfile-dom0-unpacker detects an error, it sends error report to
stdout and terminate (so stdout is closed). That close should be
transferred to the VM process (as EOF on its stdin), which will signal
it to stop sending the data and handle error report.
Also qrexec-client holds the connection until both stdin and
stdout are closed.
So when that EOF is missing, tar2qfile will not detect error report and
still tries to send the data and qrexec-client will hold the
connection while receiving process is long dead.

To prevent that deadlock from happening, close FD in python code, so
qfile-dom0-unpacker will be the last owner of write end of the pipe.
When it closes its stdout, qrexec-client will receive EOF at its stdin.

core/backup.py

@@ -1285,6 +1285,10 @@ def restore_vm_dirs (backup_source, restore_tmpdir, passphrase, vms_dirs, vms,
     # to the VM), while tar output filelist on stdout
     if appvm:
         filelist_pipe = command.stderr
+        # let qfile-dom0-unpacker hold the only open FD to the write end of
+        # pipe, otherwise qrexec-client will not receive EOF when
+        # qfile-dom0-unpacker terminates
+        vmproc.stdin.close()
     else:
         filelist_pipe = command.stdout