Use "cachevol" to refer to cache on a single LV

Use "cachevol" to refer to cache on a single LV

and "cachepool" to refer to a cache on a cache pool object.

The problem was that the --cachepool option was being used
to refer to both a cache pool object, and to a standard LV
used for caching.  This could be somewhat confusing, and it
made it less clear when each kind would be used.  By
separating them, it's clear when a cachepool or a cachevol
should be used.

Previously:

- lvm would use the cache pool approach when the user passed
  a cache-pool LV to the --cachepool option.

- lvm would use the cache vol approach when the user passed
  a standard LV in the --cachepool option.

Now:

- lvm will always use the cache pool approach when the user
  uses the --cachepool option.

- lvm will always use the cache vol approach when the user
  uses the --cachevol option.