The migration code uses unsigned values for 16, 32 and 64-bit
operations. Fix the script to do the same.

This was causing an issue when parsing the migration stream generated
on the ppc64 target because one of instance_ids was larger than the
32bit signed maximum:

Traceback (most recent call last):
  File "/home/fabiano/kvm/qemu/build/scripts/analyze-migration.py", line 658, in <module>
    dump.read(dump_memory = args.memory)
  File "/home/fabiano/kvm/qemu/build/scripts/analyze-migration.py", line 592, in read
    classdesc = self.section_classes[section_key]
KeyError: ('spapr_iommu', -2147483648)

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231009184326.15777-6-farosas@suse.de>