qemu-iotests/199: increase postcopy period

qemu-iotests/199: increase postcopy period

The test wants to force a bitmap postcopy. Still, the resulting
postcopy period is very small. Let's increase it by adding more
bitmaps to migrate. Also, test disabled bitmaps migration.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Tested-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200727194236.19551-7-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>