analyze-migration.py: fix read_migration_debug_json() return type

Since we use result of read_migration_debug_json() as JSON formatted string,
we must provide proper type. Before Python 3.6 json.loads() method
support only str typed input.

Signed-off-by: Alexey Kirillov <lekiravi@yandex-team.ru>
Message-Id: <20200715152135.20287-1-lekiravi@yandex-team.ru>
[ehabkost: added comment explaining why decode() is needed}
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>