analyze-migration.py: fix find() type error

analyze-migration.py: fix find() type error

Traceback (most recent call last):
  File "../scripts/analyze-migration.py", line 611, in <module>
    dump.read(desc_only = True)
  File "../scripts/analyze-migration.py", line 513, in read
    self.load_vmsd_json(file)
  File "../scripts/analyze-migration.py", line 556, in load_vmsd_json
    vmsd_json = file.read_migration_debug_json()
  File "../scripts/analyze-migration.py", line 89, in read_migration_debug_json
    nulpos = data.rfind("\0")
TypeError: argument should be integer or bytes-like object, not 'str'

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20191127101038.327080-2-marcandre.lureau@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>