kvm: Don't walk memory_size == 0 slots in kvm_client_migration_log

If we've unregistered a memory area, we should avoid calling
qemu_get_ram_ptr() on the left over phys_offset cruft in the
slot array.  Now that we support removing ramblocks, the
phys_offset ram_addr_t can go away and cause a lookup fault
and abort.
Signed-off-by: NAlex Williamson <alex.williamson@redhat.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>

kvm-all.c

@@ -274,6 +274,9 @@ static int kvm_set_migration_log(int enable)
     for (i = 0; i < ARRAY_SIZE(s->slots); i++) {
         mem = &s->slots[i];
 
+        if (!mem->memory_size) {
+            continue;
+        }
         if (!!(mem->flags & KVM_MEM_LOG_DIRTY_PAGES) == enable) {
             continue;
         }