s390/kvm: split kvm mem slots at 4TB

Instead of splitting at an unaligned address, we can simply split at
4TB.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Igor Mammedov <imammedo@redhat.com>

target/s390x/kvm.c

@@ -126,12 +126,11 @@
 /*
  * KVM does only support memory slots up to KVM_MEM_MAX_NR_PAGES pages
  * as the dirty bitmap must be managed by bitops that take an int as
- * position indicator. If we have a guest beyond that we will split off
- * new subregions. The split must happen on a segment boundary (1MB).
+ * position indicator. This would end at an unaligned  address
+ * (0x7fffff00000). As future variants might provide larger pages
+ * and to make all addresses properly aligned, let us split at 4TB.
  */
-#define KVM_MEM_MAX_NR_PAGES ((1ULL << 31) - 1)
-#define SEG_MSK (~0xfffffULL)
-#define KVM_SLOT_MAX_BYTES ((KVM_MEM_MAX_NR_PAGES * TARGET_PAGE_SIZE) & SEG_MSK)
+#define KVM_SLOT_MAX_BYTES (4UL * TiB)
 
 static CPUWatchpoint hw_watchpoint;
 /*