[libvirt] Add filesystem pool formatting (v4)

[Osier Yang]

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>