This adds the skip option which allows qemu-img dd to skip a number of blocks
before copying the input.

A test case was added to test the skip option.

Signed-off-by: Reda Sallahi <fullmanet@gmail.com>
Message-id: 20160810141609.32727-1-fullmanet@gmail.com
Signed-off-by: Max Reitz <mreitz@redhat.com>