virt-v2v test transfers

This tests the transfer side of virt-v2v, ie. can we transfer guest
data quickly and reliably from VMware and RHEL 5 Xen sources.  It
tests the speed and reliability of the block/curl and block/ssh
drivers in qemu especially, and also libguestfs.

Notes:

- This does not test conversion of guests.  For testing that, see
  virt-v2v-test-harness(1) and the corresponding git repositories.

- Not all the tests use virt-v2v itself.  We also use virt-inspector
  and qemu-img directly to test various aspects of the v2v transfer
  process.

- Read the fine manual (run-transfer-tests.pl --man) before starting.