p2v/v2v: List output drivers in `virt-v2v --machine-readable', and use that in virt-p2v.

The new output of virt-v2v --machine-readable now lists the input and
output drivers available in the binary:

  $ virt-v2v --machine-readable
  virt-v2v
  libguestfs-rewrite
  input:disk
  input:libvirt
  input:libvirtxml
  output:glance
  output:libvirt
  output:local
  output:rhev

With this information, we can now pre-populate the virt-p2v GUI
combo box.
v2v: Review 3-X comments in TODO file and remove any which are not short-term goals to fix.