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

[Richard W.M. Jones]

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.