conf: use virDirRead API
    
    When reading configuration files, we were silently ignoring
    directory read failures.  While unlikely, we might as well
    report them.
    
    * src/conf/domain_conf.c (virDomainObjListLoadAllConfigs): Report
    readdir errors.
    * src/conf/network_conf.c (virNetworkLoadAllState)
    (virNetworkLoadAllConfigs): Likewise.
    * src/conf/nwfilter_conf.c (virNWFilterLoadAllConfigs): Likewise.
    * src/conf/storage_conf.c (virStoragePoolLoadAllConfigs):
    Likewise.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>