While stderr isn't connected to the local_program argument (qrun-in-vm)
of qrexec-client-vm, it *is* connected (by qrexec-client-vm itself) to
the command the user passed to *to qvm-run-vm*.
qvm-run-vm cannot make a separate qubes.WaitForSession call for a
DispVM. Instead, pass the new WaitForSession argument to qubes.VMShell,
which will do the equivalent.