storage-setup: Move some path conversion code in a separate function
storage-setup: Move some path conversion code in a separate function

Move some code to do lvm to device mapper path conversion in a separate
function. No functionality change.
  
Signed-off-by: Vivek Goyal <61d11262d0d9873ddb5a887f48fcb04e8c5973d0@redhat.com>

shishir-a412ed/docker-storage-setup,thescriptbender/container-storage-setup