qga: Fix channel initialization check in run_agent_once Reviewed-by: Yan Vugenfirer <yvugenfi@redhat.com> Reviewed-by: Michal Privoznik <mprivozn@redhat.com> Link: https://lore.kernel.org/qemu-devel/20250825140549.146617-2-kkostiuk@redhat.com Signed-off-by: Kostiantyn Kostiuk <kkostiuk@redhat.com>