[libvirt] Storage backend and RBD improvements

[Wido den Hollander]

This series of patches is mainly focussed on improving the RBD
storage pool backend.

In the series it fixes a bug in the storage pool driver regarding
volume wiping and it also adds the new flag to instruct storage
pools to use TRIM for wiping volumes.

Afterwards it adds this functionality to the RBD storage pool
and in addition it also adds to ability to clone RBD volumes.

The original series of patches were reviewed by John Ferlan and
this series are revised based on his great feedback.

Wido