conf: backup: Format index of 'store'

conf: backup: Format index of 'store'

Similarly to other disk-related stuff, the index is useful when you want
to refer to the image in APIs such as virDomainSetBlockThreshold.

For internal use we also need to parse it inside of the status XML.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>