iotests: Fix up python style in 300

iotests: Fix up python style in 300

Break some long lines, and relax our type hints to be more generic to
any JSON, in order to more easily permit the additional JSON depth now
possible in migration parameters.  Detected by iotest 297.

Fixes: ca4bfec41d56
 (qemu-iotests: 300: Add test case for modifying persistence of bitmap)
Reported-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210215220518.1745469-1-eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>