qemudriver: record QEMU stderr in exception message

When starting QEMU fails, e.g. because of a wrong command line option,
its stderr is not printed by default.

Fix this to make debugging startup issues easier.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>

Author: a3f

labgrid/driver/qemudriver.py

@@ -273,9 +273,9 @@ def on(self):
             self.qmp = QMPMonitor(self._child.stdout, self._child.stdin)
         except QMPError as exc:
             if self._child.poll() is not None:
-                self._child.communicate()
+                _, err = self._child.communicate()
                 raise IOError(
-                    f"QEMU process terminated with exit code {self._child.returncode}"
+                    f"QEMU error: {err} (exitcode={self._child.returncode})"
                 ) from exc
             raise