Ignore thin pool LVM devices.

This should resolve:

  https://bugzilla.redhat.com/show_bug.cgi?id=924672

For BZ 924672 the problem stems from the fact that thin pool logical
volume devices show up in /sbin/lvs output just like normal logical
volumes do. Libvirt incorrectly assumes they are just normal logical
volumes and that they will have a corresponding /dev/vgname/lvname
device that has been created by udev and tries to use this device.

To illustrate here is an example of the /dev/vgname/ directory and
the lvs output for a normal lv, thin lv, and thin pool:

    LV     VG       Attr      LSize  Pool Origin Data%  Move Log Copy%  Convert
    lv     vgguests -wi-a----  1.00g
    pool   vgguests twi-a-tz- 11.00g               0.00
    thinlv vgguests Vwi-a-tz-  1.00g pool          0.00
total 0
lrwxrwxrwx. 1 root root 7 Oct  8 19:35 lv -> ../dm-7
lrwxrwxrwx. 1 root root 7 Oct  8 19:37 thinlv -> ../dm-6

This patch modifies virStorageBackendLogicalMakeVol() to ignore thin pool
devices.

src/storage/storage_backend_logical.c

@@ -85,6 +85,14 @@ virStorageBackendLogicalMakeVol(virStoragePoolObjPtr pool,
     if (attrs[4] != 'a')
         return 0;
 
+    /*
+     * Skip thin pools(t). These show up in normal lvs output
+     * but do not have a corresponding /dev/$vg/$lv device that
+     * is created by udev. This breaks assumptions in later code.
+     */
+    if (attrs[0] == 't')
+        return 0;
+
     /* See if we're only looking for a specific volume */
     if (data != NULL) {
         vol = data;