conf: fix backing store parse off-by-one
    
    Commit 546154e parses the type attribute from a <backingStore>
    element, but forgot that the earlier commit 9673418 added a
    placeholder element in the same 1.2.3 release; as a result,
    the C code was mistakenly allowing "none" as a type.
    
    Similarly, the same commit allows "none" as the <format>
    sub-element type, even though that has been a placeholder
    since the 0.10.2 release with commit f772b3d.
    
    * src/conf/domain_conf.c (virDomainDiskBackingStoreParse): Require
    non-zero types.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>