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 235e8982ad 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>
