[libvirt] Add filesystem pool formatting (v4)

This patch series are based on Dave Allan's previous posts: https://www.redhat.com/archives/libvir-list/2010-June/msg00040.html I rebased them and make changes per last feedback from Eric, and also other some small changes. <QUOTE> The following patches add the ability to format filesystem pools when the appropriate flags are passed to pool build. As before, I have implemented two new flags: VIR_STORAGE_POOL_BUILD_NO_OVERWRITE causes the build to probe for an existing pool of the requested type. The build operation formats the filesystem if it does not find an existing filesystem of that type. VIR_STORAGE_POOL_BUILD_OVERWRITE causes the build to format unconditionally. These patches incorporate all the feedback received on earlier versions. The incremental is pretty unhelpful, so these are complete replacement patches. Dave </QUOTE>