iotests.py: QemuIoInteractive: print output on failure

Make it simpler to debug when qemu-io fails due to wrong arguments or
environment.

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Eric Blake <[email protected]>

Author: Vladimir Sementsov-Ogievskiy

tests/qemu-iotests/iotests.py

@@ -217,7 +217,13 @@ def __init__(self, *args):
                                    stdout=subprocess.PIPE,
                                    stderr=subprocess.STDOUT,
                                    universal_newlines=True)
-        assert self._p.stdout.read(9) == 'qemu-io> '
+        out = self._p.stdout.read(9)
+        if out != 'qemu-io> ':
+            # Most probably qemu-io just failed to start.
+            # Let's collect the whole output and exit.
+            out += self._p.stdout.read()
+            self._p.wait(timeout=1)
+            raise ValueError(out)
 
     def close(self):
         self._p.communicate('q\n')