[Libguestfs] [PATCH nbdkit 0/6] cow: Implement trimming and extents.

This patch series implements trimming and extents in nbdkit-cow-filter. The aim of this is to have feature parity with qcow2 zero clusters, which we can then use to replace the qcow2 overlays currently used to protect the source disk in virt-v2v. First 3 patches are some preparatory enhancements/clean-up. To implement extents: We iterate over the blocks in the filter, if the block is allocated (ie. contains a write which has been saved in the overlay) then the whole block is allocated data, else we have to ask the underlying plugin for the information about that block. (This ends up making lots of small extents calls into the plugin, which could be optimized in future.) To implement trim: We extend the overlay bitmap with an extra bit per block so we can store whether the block has been trimmed. A future optimization is to actually trim the overlay (it only saves space, it's not needed for correctness), but this patch series does not do this. Final patch implements a couple of tests. Rich.