conf: Don't free the constructed string in virDomainGetBlkioParametersAssignFromDef
    
    virTypedParameterAssign steals the string rather than copying it into
    the typed parameter and thus freeing it leads to a crash when attempting
    to serialize the results.
    
    This was introduced in commit 9f50f6e2 and later made an universal
    helper in 32e6339c.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1351473