dmstats: use canonical path when reporting errors

dmstats: use canonical path when reporting errors

When a 'dmstats create --filemap' operation fails (e.g. during
open(2), close(2), or dm_stats_create_regions_from_fd()), use the
canonical version of the path. This avoids cryptic/confusing error
messages when symbolic links exist in the path argument given:

  # findmnt /var/lib/libvirt/images -otarget,source
  TARGET                  SOURCE
  /var/lib/libvirt/images /dev/mapper/vg_hex-lv_images

  # readlink /var/lib/libvirt/images/my.img
  /boot/my.img

  # dmstats create --filemap /var/lib/libvirt/images/my.img
  Cannot map file: not a device-mapper device.
  Could not create regions from file /var/lib/libvirt/images/my.img
  Command failed

Using the canonical path the error is immediately obvious:

  # dmstats create --filemap /var/lib/libvirt/images/my.img
  Cannot map file: not a device-mapper device.
  Could not create regions from file /boot/my.img
  Command failed