migration: Rename vmstate_info_nullptr

Rename vmstate_info_nullptr from "uint64_t" to "nullptr". This vmstate
actually reads and writes just a byte, so the proper name would be
uint8. However, since this is a marker for a NULL pointer, it's
convenient to have a more explicit name that can be identified by the
consumers of the JSON part of the stream.

Change the name to "nullptr" and add support for it in the
analyze-migration.py script. Arbitrarily use the name of the type as
the value of the field to avoid the script showing 0x30 or '0', which
could be confusing for readers.

Reviewed-by: Peter Xu <peterx@redhat.com>
Message-Id: <20250109185249.23952-5-farosas@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
(cherry picked from commit f52965bf0eeee28e89933264f1a9dbdcdaa76a7e)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

migration/vmstate-types.c

@@ -338,7 +338,7 @@ static int put_nullptr(QEMUFile *f, void *pv, size_t size,
 }
 
 const VMStateInfo vmstate_info_nullptr = {
-    .name = "uint64",
+    .name = "nullptr",
     .get  = get_nullptr,
     .put  = put_nullptr,
 };

scripts/analyze-migration.py

@@ -417,6 +417,28 @@ class VMSDFieldIntLE(VMSDFieldInt):
         super(VMSDFieldIntLE, self).__init__(desc, file)
         self.dtype = '<i%d' % self.size
 
+class VMSDFieldNull(VMSDFieldGeneric):
+    NULL_PTR_MARKER = b'0'
+
+    def __init__(self, desc, file):
+        super(VMSDFieldNull, self).__init__(desc, file)
+
+    def __repr__(self):
+        # A NULL pointer is encoded in the stream as a '0' to
+        # disambiguate from a mere 0x0 value and avoid consumers
+        # trying to follow the NULL pointer. Displaying '0', 0x30 or
+        # 0x0 when analyzing the JSON debug stream could become
+        # confusing, so use an explicit term instead.
+        return "nullptr"
+
+    def __str__(self):
+        return self.__repr__()
+
+    def read(self):
+        super(VMSDFieldNull, self).read()
+        assert(self.data == self.NULL_PTR_MARKER)
+        return self.data
+
 class VMSDFieldBool(VMSDFieldGeneric):
     def __init__(self, desc, file):
         super(VMSDFieldBool, self).__init__(desc, file)
@@ -558,6 +580,7 @@ vmsd_field_readers = {
     "bitmap" : VMSDFieldGeneric,
     "struct" : VMSDFieldStruct,
     "capability": VMSDFieldCap,
+    "nullptr": VMSDFieldNull,
     "unknown" : VMSDFieldGeneric,
 }