tests/qemu-iotests: explicitly invoke 'check' via 'python'

tests/qemu-iotests: explicitly invoke 'check' via 'python'

The 'check' script will use "#!/usr/bin/env python3" by default
to locate python, but this doesn't work in distros which lack a
bare 'python3' binary like NetBSD.

We need to explicitly invoke 'check' by referring to the 'python'
variable in meson, which resolves to the detected python binary
that QEMU intends to use.

This fixes a regression introduced by

  commit 51ab5f8bd795d8980351f8531e54995ff9e6d163
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Wed Mar 15 17:43:23 2023 +0000

    iotests: register each I/O test separately with meson

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230329124539.822022-1-berrange@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230403134920.2132362-9-alex.bennee@linaro.org>