In preparation to allow heterogenous backups store the 'incremental'
field per-disk and fill it by default from the per-backup field.

Having this will be important once we'll want to allow incremental
backup working while hotplugging a new disk.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>