s390x: introduce s390_get_memory_limit()

s390x: introduce s390_get_memory_limit()

Let's add s390_get_memory_limit(), to query what has been successfully
set via s390_set_memory_limit(). Allow setting the limit only once.

We'll remember the limit in the machine state. Move
s390_set_memory_limit() to machine code, merging it into
set_memory_limit(), because this really is a machine property.

Message-ID: <20241219144115.2820241-7-david@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>