s390x: Fix CSS migration

s390x: Fix CSS migration

Commit a55ae46683 ("s390: move css_migration_enabled from machine to
css.c") disabled CSS migration globally instead of doing it
per-instance.

CC: Paolo Bonzini <pbonzini@redhat.com>
CC: qemu-stable@nongnu.org #9.1
Fixes: a55ae46683 ("s390: move css_migration_enabled from machine to css.c")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2704
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20250109185249.23952-8-farosas@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
(cherry picked from commit c76ee1f6255c3988a9447d363bb17072f1ec84e1)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>