devmapper: Add blkdiscard option and disable it on raw devices

The blkdiscard hack we do on container/image delete is pretty slow, but
required to restore space to the "host" root filesystem. However, it
is pretty useless on raw devices, and you may not need it in development
either.

In a simple test of the devicemapper backend on loopback the time to delete
20 container went from 11 seconds to 0.4 seconds with -o blkdiscard=false.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)