[Libguestfs] p2v: Allow virt-v2v input and output drivers containing '-' (RHBZ#1590220).

v1 was here: https://www.redhat.com/archives/libguestfs/2018-June/msg00047.html v1 -> v2: - Add a regular expression in the virt-v2v modules code too. - - - I was planning a much more ambitious second version of this patch (half written too) which had: virt-v2v --describe-modules that listed a big chunk of XML which virt-p2v would parse. These would include the names of the available modules and the long descriptions (currently hard-coded into virt-p2v p2v/gui.c). However it was a bunch of tricky code and it would be difficult to maintain interop with both old and new versions of virt-v2v. Also there's a problem with translating strings. So in the end I wasn't happy with it. Rich.