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: NEric Blake <eblake@redhat.com>