Snapshots and block-copy have a flag that forces qemu to re-use existing
file. Our docs weren't exactly clear on what the existing file should
contain for this to actually work.

Re-word the docs a bit to state that the file needs to be pre-created in
the desired format and the backing chain metadata needs to be set prior
to handing it over to qemu.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1084360