/dev/mapper paths should not be returned from C inspection APIs (RHBZ#638899).

With this patch, /dev/mapper paths do not appear in the output
of guestfs_inspect_os, as you can see from this example:

Welcome to guestfish, the libguestfs filesystem interactive shell for
editing virtual machine filesystems.

Type: 'help' for a list of commands
      'man' to read the manual
      'quit' to quit the shell

Operating system: Fedora release 13 (Goddard)
/dev/vg_f13x64/lv_root mounted on /              <--- NB
/dev/vda1 mounted on /boot