tests/functional: set 'qemu_bin' as an object level field

tests/functional: set 'qemu_bin' as an object level field

The 'qemu_bin' field is currently set on the class, despite being
accessed as if it were an object instance field with 'self.qemu_bin'.

This is no obvious need to have it as a class field, so move it into
the object instance.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20250228102738.3064045-3-berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>