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>