conf: add support for discard_granularity

conf: add support for discard_granularity

This introduces the ability to set the discard granularity option
for a disk.  It defines the smallest amount of data that can be
discarded in a single operation (useful for managing and
optimizing storage).

However, most hypervisors automatically set the proper discard
granularity and users usually do not need to change the default
setting.

Signed-off-by: Kristina Hanicova <khanicov@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>