tests: add blkdiscard offsets test

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
blkdiscard: fail on sector misalignment
blkdiscard: add support for steps and progress

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
blkdiscard: update man page

Signed-off-by: Karel Zak <kzak@redhat.com>