Make sure passed in block devices are valid block devices
Make sure passed in block devices are valid block devices

Right now one can pass in any xyz string in DEVS= and there are no error
checks and sfdisk tries to partition it and fails with odd error messages
like as follows.

INFO: Volume group backing root filesystem could not be determined
./docker-storage-setup.sh: line 463: * 2 - 2048 : syntax error: operand expected (error token is "* 2 - 2048 ")
ERROR: No valid volume group found. Exiting.

Instead check for valid block devices early and fail gracefully.

Signed-off-by: Vivek Goyal <61d11262d0d9873ddb5a887f48fcb04e8c5973d0@redhat.com>

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