[libvirt] Storage backend and RBD improvements

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