memory: assign MemoryRegionOps to all regions

This allows to remove the checks on section->readonly.  Simply,
write accesses to ROM will not be considered "direct" and will
go through mr->ops without any special intervention.
Reviewed-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

memory.c

@@ -788,7 +788,8 @@ void memory_region_init(MemoryRegion *mr,
                         const char *name,
                         uint64_t size)
 {
-    mr->ops = NULL;
+    mr->ops = &unassigned_mem_ops;
+    mr->opaque = NULL;
     mr->parent = NULL;
     mr->size = int128_make64(size);
     if (size == UINT64_MAX) {