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

include/linux/kvm_host.h

@@ -65,6 +65,7 @@
 #define KVM_PFN_ERR_FAULT	(KVM_PFN_ERR_MASK)
 #define KVM_PFN_ERR_HWPOISON	(KVM_PFN_ERR_MASK + 1)
 #define KVM_PFN_ERR_BAD		(KVM_PFN_ERR_MASK + 2)
+#define KVM_PFN_ERR_RO_FAULT	(KVM_PFN_ERR_MASK + 3)
 
 static inline bool is_error_pfn(pfn_t pfn)
 {