p2v: Ensure we are using virt-v2v >= 1.28.

Must NOT use:

- One of the interim versions of virt-v2v (1.27.x) that we published
  during development.

- Old virt-v2v (0.9.x).

Also use the --version option (instead of -V) since old virt-v2v
required it:

  $ virt-v2v -V
  Option v is ambiguous (version, vmtype)
  Usage:
  [...]
  $ virt-v2v --version
  0.9.1
builder: Document that --edit option can be split across lines.