[libvirt] Improvements to the RBD storage backend

[Wido den Hollander]

Hi,

After review by John Ferlan I'm sending a revised version of my earlier
patch which adds the fast-diff feature for RBD volumes.

Before the patch there is one small improvement which is later used
by the fast-diff feature.

The other is a bugfix where the #ifdef in the code wasn't requiring the
right version of librbd.

Wido