iotests: ensure we print nbd server log on error

If we abort the iotest early the server.log file might contain useful
information for diagnosing the problem. Ensure its contents are
displayed in this case.
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>
Message-Id: <20190220145819.30969-2-berrange@redhat.com>
[eblake: fix shell quoting]
Signed-off-by: NEric Blake <eblake@redhat.com>

tests/qemu-iotests/233

@@ -30,6 +30,8 @@ _cleanup()
 {
     nbd_server_stop
     _cleanup_test_img
+    # If we aborted early we want to see this log for diagnosis
+    test -f "$TEST_DIR/server.log" && cat "$TEST_DIR/server.log"
     rm -f "$TEST_DIR/server.log"
     tls_x509_cleanup
 }
@@ -120,6 +122,7 @@ $QEMU_IO -f $IMGFMT -r -U -c 'r -P 0x22 1m 1m' "$TEST_IMG" | _filter_qemu_io
 echo
 echo "== final server log =="
 cat "$TEST_DIR/server.log"
+rm -f "$TEST_DIR/server.log"
 
 # success, all done
 echo "*** done"