[Libguestfs] [PATCH v2v] convert: Allow preferred block driver to be specified on the command line

This is just an outline patch, only compile tested. It doesn't make changes to virt-v2v-in-place, but those would be the same as made in v2v/v2v.ml. It reuses the existing Types.guestcaps_block_type which is a bit ugly but fairly practical. I've made the change to the documentation, but it needs a test. Rich.