Clean up check for minimum backing storage of pool
Clean up check for minimum backing storage of pool

Add a public function for minimum size, and use it.

Use bytes() function to convert Sectors to Bytes.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

stratis-storage/stratisd,mulkieran/stratisd,trgill/stratisd,stratis-storage/stratisd,trgill/stratisd,mulkieran/stratisd,stratis-storage/stratisd