fix double free the memslot in kvm_set_phys_mem

Luiz Capitulino reported that guest refused to boot and qemu
complained with:
kvm_set_phys_mem: error unregistering overlapping slot: Invalid argument

It is caused by commit 235e8982 that did double free for the memslot
so that the second one raises the -EINVAL error

Fix it by reset memory size only if it is needed

Reported-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

kvm-all.c

@@ -206,7 +206,8 @@ static int kvm_set_user_memory_region(KVMState *s, KVMSlot *slot)
     if (s->migration_log) {
         mem.flags |= KVM_MEM_LOG_DIRTY_PAGES;
     }
-    if (mem.flags & KVM_MEM_READONLY) {
+
+    if (slot->memory_size && mem.flags & KVM_MEM_READONLY) {
         /* Set the slot size to 0 before setting the slot to the desired
          * value. This is needed based on KVM commit 75d61fbc. */
         mem.memory_size = 0;