Ignore thin pool LVM devices.

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.