For guests backed by gluster volumes (or other network storage) we don't
fill the backing chain (see qemuDomainDetermineDiskChain). This leaves
the "relPath" field of the top image NULL. This causes a crash in
virStorageFileChainLookup() when looking up a backing element for such a
disk.

Since I'm working on adding support for network storage and one of the
steps will make the "relPath" field optional let's use STREQ_NULLABLE
instead of STREQ in virStorageFileChainLookup() to avoid the problem.