Failure while executing KTar::open while using KCompressionDevice as the device

[Nick Shaforostoff]

 > Aleix suggested QBuffer, but archives can be *huge*, so this might
> eat all available memory. This is the reason why KTar uses a temp file
> for the compressed-tar case.

qbuffer may be used for decompressing archives that are e.g. less than 50% of free ram
if an archive is larger then tmpfile-based branch would be executed.