kvm: return -EFAULT for writes to !VM_WRITE IO mappings

hva_to_pfn() 对不可写 VMA 统一返回 KVM_PFN_ERR_FAULT，不再让 PFNMAP 映射产生 KVM_EXIT_MMIO；KVM_EXIT_MMIO 仅限显式只读 memslot。