'qvm-run --dispvm' cannot easily make a separate qubes.WaitForSession
call. Instead, if --gui is active, pass the new WaitForSession argument
to qubes.VMShell, which will do the equivalent.
The unit tests have been copied (in slightly adapted form) from commit
a620f02e2aFixesQubesOS/qubes-issues#3012ClosesQubesOS/qubes-core-admin-client#49