At the end of disk image installs, use fstrim on the generated filesystem to
discard any blocks that were allocated during the install and are now unused.
This will allow tools such as qemu-img to create images that do not include
deleted data.
For raw disk images that do not go through qemu-img, use fallocate --dig-holes
to create sparse holes in place of the unused blocks.
(cherry picked from commit 9717b3fd98)
Related: rhbz#1628645
Related: rhbz#1628646
Related: rhbz#1628647
Related: rhbz#1628648
c13aa84f02