doc: Fix doc for backingStore

doc: Fix doc for backingStore

I spent quite some time figuring that backingStore info
isn't included in the dom xml, unless guest is up and
running. Hopefully putting that in the doc should help.

Also, several people have complained that libvirt reports
a backing file as raw, even though they expected it to be
qcow2; where the culprit is usually the user forgetting to
create the file with qemu-img create -o backing_fmt=qcow2.

This patch adds that info to the doc.

Signed-off-by: Deepak C Shetty <deepakcs@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>