s390x/pv: prepare for memory devices

s390x/pv: prepare for memory devices

Let's avoid checking for the maxram_size, and instead rely on the memory
limit determined in s390_memory_init(), that might be larger than
maxram_size, for example due to alignment purposes.

This check now correctly mimics what the kernel will check in
kvm_s390_pv_set_aside(), whereby a VM <= 2 GiB VM would end up using
a segment type ASCE.

Message-ID: <20241219144115.2820241-12-david@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>