-i ova: Tidy up the description in the manual page.

This updates commit 90b1e9845fa0a25de12587749f259d0da8d70a01.
v2v: Move json-generating code into separate module.

Just code motion so we can reuse the same code for making
`json:' remote SSH URLs.
v2v: -i libvirtxml: Detect disk format using any <driver type=...> field.

Previously we only looked for <driver name=qemu type=...>.  However
for Xen imports, the name field could be name=tap (or other values in
fact).  Since there can only be one <driver/> element under <disk/>,
look for any <driver type=...> in order to get the disk format.

A further complication for Xen is that type=aio means "raw" (for
obscure historical reasons), so map that.

Thanks: Dan Berrangé.
v2v: Add support for access to remote RHEL 5 Xen hosts over ssh.
v2v: docs: Clarify how ESX & Xen imports happen through libvirt.