conf: Add a formatting macro for all the blkiotune values

conf: Add a formatting macro for all the blkiotune values

Rather than copy-paste - use a macro

Unfortunately due to how the RNG schema was written keeping the 'value'
and 'value'_max next to each other in the XML causes a schema failure,
so the FORMAT has to write out singly rather than optimizing to write
out both values at once

Signed-off-by: John Ferlan <jferlan@redhat.com>