migration: Fix arrays of pointers in JSON writer

migration: Fix arrays of pointers in JSON writer

Currently, if an array of pointers contains a NULL pointer, that
pointer will be encoded as '0' in the stream. Since the JSON writer
doesn't define a "pointer" type, that '0' will now be an uint8, which
is different from the original type being pointed to, e.g. struct.

(we're further calling uint8 "nullptr", but that's irrelevant to the
issue)

That mixed-type array shouldn't be compressed, otherwise data is lost
as the code currently makes the whole array have the type of the first
element:

css = {NULL, NULL, ..., 0x5555568a7940, NULL};

{"name": "s390_css", "instance_id": 0, "vmsd_name": "s390_css",
 "version": 1, "fields": [
    ...,
    {"name": "css", "array_len": 256, "type": "nullptr", "size": 1},
    ...,
]}

In the above, the valid pointer at position 254 got lost among the
compressed array of nullptr.

While we could disable the array compression when a NULL pointer is
found, the JSON part of the stream still makes part of downtime, so we
should avoid writing unecessary bytes to it.

Keep the array compression in place, but if NULL and non-NULL pointers
are mixed break the array into several type-contiguous pieces :

css = {NULL, NULL, ..., 0x5555568a7940, NULL};

{"name": "s390_css", "instance_id": 0, "vmsd_name": "s390_css",
 "version": 1, "fields": [
     ...,
     {"name": "css", "array_len": 254, "type": "nullptr", "size": 1},
     {"name": "css", "type": "struct", "struct": {"vmsd_name": "s390_css_img", ... }, "size": 768},
     {"name": "css", "type": "nullptr", "size": 1},
     ...,
]}

Now each type-discontiguous region will become a new JSON entry. The
reader should interpret this as a concatenation of values, all part of
the same field.

Parsing the JSON with analyze-script.py now shows the proper data
being pointed to at the places where the pointer is valid and
"nullptr" where there's NULL:

"s390_css (14)": {
    ...
    "css": [
        "nullptr",
        "nullptr",
        ...
        "nullptr",
        {
            "chpids": [
            {
                "in_use": "0x00",
                "type": "0x00",
                "is_virtual": "0x00"
            },
            ...
            ]
        },
        "nullptr",
    }

Reviewed-by: Peter Xu <peterx@redhat.com>
Message-Id: <20250109185249.23952-7-farosas@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>