libblkid: add blkid_probe_open_device()

Add a convenience function that opens a device and assigns it to
the probe, using VFS operations if previously set. This enables
the composable pattern:

  blkid_new_probe()
  → blkid_probe_set_vfs()
  → blkid_probe_open_device()

Refactor blkid_new_probe_from_filename() to use it internally.

Addresses: https://github.com/util-linux/util-linux/issues/4308
Signed-off-by: Karel Zak <kzak@redhat.com>