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>