drivers: use virDirRead API

drivers: use virDirRead API

Convert all remaining clients of readdir to use the new
interface, so that we can ensure (unlikely) errors while
reading a directory are reported.

* src/openvz/openvz_conf.c (openvzAssignUUIDs): Use new
interface.
* src/parallels/parallels_storage.c (parallelsFindVolumes)
(parallelsFindVmVolumes): Report readdir failures.
* src/qemu/qemu_driver.c (qemuDomainSnapshotLoad): Ignore readdir
failures.
* src/secret/secret_driver.c (loadSecrets): Likewise.
* src/qemu/qemu_hostdev.c
(qemuHostdevHostSupportsPassthroughVFIO): Report readdir failures.
* src/xen/xen_inotify.c (xenInotifyOpen): Likewise.
* src/xen/xm_internal.c (xenXMConfigCacheRefresh): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>