migration: Rename vmstate_info_nullptr

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>