KVM: introduce KVM_PFN_ERR_RO_FAULT

In the later patch, it indicates failure when we try to get a writable
pfn from the readonly memslot

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>