[PATCH RFC 5/6] iotests: use pyvenv/bin/python3 to launch child test processes

[John Snow <jsnow@redhat.com>]

Now that there's a fancy venv set up for us by configure, we should take
care to use it even when check is invoked directly.

./check will now use the pyvenv environment when launching python tests,
which allows those tests to find and access the 'qemu.*' packages
without PYTHONPATH modifications.

RFC: This patch now means that ./check may launch test subprocesses
using a different Python than the one used to launch it. If that isn't
acceptable, we might need a launcher shim whose job it is to sit above
"check" and just chooses the correct Python.

...Or maybe it's fine the way it is.

Comments welcome, sorry for my indecision.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 tests/qemu-iotests/testenv.py | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/tests/qemu-iotests/testenv.py b/tests/qemu-iotests/testenv.py
index e67ebd254b..1b095d70f2 100644
--- a/tests/qemu-iotests/testenv.py
+++ b/tests/qemu-iotests/testenv.py
@@ -138,7 +138,20 @@ def init_binaries(self) -> None:
              PYTHON (for bash tests)
              QEMU_PROG, QEMU_IMG_PROG, QEMU_IO_PROG, QEMU_NBD_PROG, QSD_PROG
         """
-        self.python = sys.executable
+        # The python we want to use to launch tests.
+        self.python: str = str(
+            Path(self.build_root).joinpath('pyvenv', 'bin', 'python3')
+        )
+        # RFC: Do I need to amend '.exe' for windows, or nah?
+
+        if self.python != sys.executable:
+            print(
+                "Note: "
+                f"check was launched with a Python ({sys.executable}) "
+                f"that doesn't match QEMU's configured Python ({self.python})."
+                " QEMU's Python will be used for individual test processes.",
+                file=sys.stderr
+            )
 
         def root(*names: str) -> str:
             return os.path.join(self.build_root, *names)
-- 
2.40.1