esx: Use MD5 sum of mount path as storage pool UUID

esx: Use MD5 sum of mount path as storage pool UUID

With the previous storage pool UUID source not all storage pools
had a proper UUID, especially GSX storage pools. The mount path
is unique per host and cannot change during the lifetime of the
datastore. Therefore, it's MD5 sum can be used as UUID.

Use gnulib's crypto/md5 module to generate the MD5 sum.