import: always use the same buffer size

import: always use the same buffer size

Let's synchronize the buffer sizes used when passing around the disk
images, i.e. size both our internal buffers and the pipe buffers the
same (so that we can always write()/read() everything in one gone -
except for the noise compression inserts).

Let's also increase the buffer sizes from 16K to 128K, which made a
difference for me, because it reduces the number of syscalls quite a
bit.