Make an exception in vm.start() actually interrupt the test. The
asyncio.wait() returns list of completed tasks, where exception may be
stored - but is not raised directly. Change to asyncio.gather() that will
propagate the exception by default.
As a side effect, avoid deprecated direct coroutine passing to
asyncio.wait(). This functionality in asyncio.gather() is not
deprecated.
qrexec-client-vm may return earlier than it's child process (it exits
right away, without waiting for its child). Add a small wait before
reading exit code from a file.
