qemu: Refactor parsing of block device IO tuning parameters.

qemu: Refactor parsing of block device IO tuning parameters.

This patch refactors the JSON parsing function that extracts the block
IO tuning parameters from qemu's output. The most impacting change
concerns the error message that is returned if the reply from qemu does
not contain the needed data. The data for IO parameter tuning were added
in qemu 1.1 and the previous error message was confusing.

This patch also breaks long lines and extracts a multiple time used code
pattern to a macro.