_console_interaction reads data from the console even when there is only
an input string to send, and no output data to wait on. This can cause
lines to be missed by wait_for_console_pattern calls that follows an
exec_command. Fix this by not reading the console if there is no pattern
to wait for.

This does not seem to affect any upstream test case that I can see, but
it does cause occasional hangs in the proposed ppc_hv_tests.py test,
usually when run on KVM hosts that are fast enough to output important
lines early enough to be consumed.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 tests/avocado/avocado_qemu/__init__.py | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/tests/avocado/avocado_qemu/__init__.py b/tests/avocado/avocado_qemu/__init__.py
index 304c428168..5a3ae7f6b5 100644
--- a/tests/avocado/avocado_qemu/__init__.py
+++ b/tests/avocado/avocado_qemu/__init__.py
@@ -144,6 +144,13 @@ def _console_interaction(test, success_message, failure_message,
             vm.console_socket.sendall(send_string.encode())
             if not keep_sending:
                 send_string = None # send only once
+
+        # Only consume console output if waiting for something
+        if success_message is None and failure_message is None:
+            if send_string is None:
+                break
+            continue
+
         try:
             msg = console.readline().decode().strip()
         except UnicodeDecodeError:
-- 
2.42.0