tests/functional: Require 'user' netdev for ppc64 e500 test

tests/functional: Require 'user' netdev for ppc64 e500 test

When commit 72cdd672e18c extended the ppc64 e500 test to add network
support, it forgot to require the 'user' netdev backend. Fix that.

Fixes: 72cdd672e18c ("tests/functional: Replace the ppc64 e500 advent calendar test")
Signed-off-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250308071328.193694-1-clg@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>