v2v: -i libvirt: Check the domain is not running (RHBZ#1138586).

It will print an error like this:

  virt-v2v: error: internal error: invalid argument: libvirt domain 'windows'
  is running, it must be shut down in order to perform virt-v2v conversion

This only works for libvirt domains, and is explicitly disabled for
test:/// URIs since these domains are always "running", but we use
these URIs for testing.