util: use virDirRead API

util: use virDirRead API

In making the conversion to the new API, I fixed a couple bugs:
virSCSIDeviceGetSgName would leak memory if a directory
unexpectedly contained multiple entries;
virNetDevTapGetRealDeviceName could report a spurious error
from a stale errno inherited before starting the readdir search.

The decision on whether to store the result of virDirRead into
a variable is based on whether the end of the loop falls through
to cleanup code automatically.  In some cases, we have loops that
are documented to return NULL on failure, and which raise an
error on most failure paths but not in the case where the directory
was unexpectedly empty; it may be worth a followup patch to
explicitly report an error if readdir was successful but the
directory was empty, so that a NULL return always has an error set.

* src/util/vircgroup.c (virCgroupRemoveRecursively): Use new
interface.
(virCgroupKillRecursiveInternal, virCgroupSetOwner): Report
readdir failures.
* src/util/virfile.c (virFileLoopDeviceOpenSearch)
(virFileNBDDeviceFindUnused, virFileDeleteTree): Use new
interface.
* src/util/virnetdevtap.c (virNetDevTapGetRealDeviceName):
Properly check readdir errors.
* src/util/virpci.c (virPCIDeviceIterDevices)
(virPCIDeviceFileIterate, virPCIGetNetName): Report readdir
failures.
(virPCIDeviceAddressIOMMUGroupIterate): Use new interface.
* src/util/virscsi.c (virSCSIDeviceGetSgName): Report readdir
failures, and avoid memory leak.
(virSCSIDeviceGetDevName): Report readdir failures.
* src/util/virusb.c (virUSBDeviceSearch): Report readdir
failures.
* src/util/virutil.c (virGetFCHostNameByWWN)
(virFindFCHostCapableVport): Report readdir failures.

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