docs: use the pyvenv version of Meson

The version in the system might be too old for QEMU; this will be
especially true if Rust is going to be enabled by default.

Adjust the docs to suggest using pyvenv/bin/meson, which is in fact
what the "make" wrappers will be running internally.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/devel/build-system.rst

@@ -450,7 +450,7 @@ are run with ``make bench``.  Meson test suites such as ``unit`` can be ran
 with ``make check-unit``, and ``make check-tcg`` builds and runs "non-Meson"
 tests for all targets.
 
-If desired, it is also possible to use ``ninja`` and ``meson test``,
+If desired, it is also possible to use ``ninja`` and ``pyvenv/bin/meson test``,
 respectively to build emulators and run tests defined in meson.build.
 The main difference is that ``make`` needs the ``-jN`` flag in order to
 enable parallel builds or tests.

docs/devel/rust.rst

@@ -66,7 +66,7 @@ __ https://mesonbuild.com/Commands.html#devenv
 As shown above, you can use the ``--tests`` option as usual to operate on test
 code.  Note however that you cannot *build* or run tests via ``cargo``, because
 they need support C code from QEMU that Cargo does not know about.  Tests can
-be run via ``meson test`` or ``make``::
+be run via Meson (``pyvenv/bin/meson test``) or ``make``::
 
    make check-rust
 

docs/system/devices/igb.rst

@@ -54,7 +54,7 @@ directory:
 
 .. code-block:: shell
 
-  meson test qtest-x86_64/qos-test
+  pyvenv/bin/meson test qtest-x86_64/qos-test
 
 ethtool can test register accesses, interrupts, etc. It is automated as an
 functional test and can be run from the build directory with the following