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

[Richard W.M. Jones]

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.