qemu: Always re-detect backing chain
    
    Since 363e9a68 we track backing chain metadata when creating snapshots
    the right way even for the inactive configuration. As we did not yet
    update other code paths that modify the backing chain (blockpull) the
    newDef backing chain gets out of sync.
    
    After stopping of a VM the new definition gets copied to the next start
    one. The new VM then has incorrect backing chain info. This patch
    switches the backing chain detector to always purge the existing backing
    chain and forces re-detection to avoid this issue until we'll have full
    backing chain tracking support.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1144922