vmware.vm.vfs.fs.size[] collects data of non-existent file system

Steps to reproduce:

1. Specify not existing mount point for parameter <fsname> of vmware.vm.vfs.fs.size[].
   e.g.
   vmware.vm.vfs.fs.size[\{$URL},\{HOST.HOST},/foo,free]
   2. Check history data of the item on the web-frontend.

Result:
Item collects history data.

Expected:
Item becomes not supported and show errror.

The item might collect the last one's value if fsname does not match any file systems, in the current implementation.

See attached patch file.