dmstats: accept multiple arguments to --filemap

dmstats: accept multiple arguments to --filemap

Make the --filemap switch take no arguments and instead accept one
or more files on the command line to be mapped and placed into
groups.

This allows --filemap to be used with a glob:

  # dmstats create --filemap *
  rhel5.10-1.qcow2: Created new group with 87 region(s) as group ID 1564.
  rhel5.10.qcow2: Created new group with 8 region(s) as group ID 1651.
  rhel7.0-1.qcow2: Created new group with 11 region(s) as group ID 1659.
  rhel7.0.qcow2: Created new group with 1454 region(s) as group ID 1670.
  vm.img: Created new group with 2 region(s) as group ID 3124.