migration: fix analyze-migration.py script with radix table

Since commit 3a38429748 ("Add a "no HPT" encoding to HTAB migration stream")
the HTAB migration stream contains a header set to "-1", meaning there
is no HPT. Teach analyze-migration.py to ignore the section in this case.

Without this fix, the script fails with a dump from a POWER9 guest:

Traceback (most recent call last):
  File "./qemu/scripts/analyze-migration.py", line 602, in <module>
    dump.read(dump_memory = args.memory)
  File "./qemu/scripts/analyze-migration.py", line 539, in read
    section.read()
  File "./qemu/scripts/analyze-migration.py", line 250, in read
    self.file.readvar(n_valid * self.HASH_PTE_SIZE_64)
  File "./qemu/scripts/analyze-migration.py", line 64, in readvar
    raise Exception("Unexpected end of %s at 0x%x" % (self.filename, self.file.tell()))
Exception: Unexpected end of migrate.dump at 0x1d4763ba

Fixes: 3a38429748 ("Add a "no HPT" encoding to HTAB migration stream")
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Juan Quintela <quintela@redhat.com>
