qemu: Use virStorageSourceIsEmpty in qemuDomainBlockCommit

The block commit API checked 'disk->src->path' to see whether there
is a reasonable disk source to be committed. As the top image can be
e.g. backed by NBD the check is not good enough. Replace it by
virStorageSourceIsEmpty.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/qemu/qemu_driver.c

@@ -17979,7 +17979,7 @@ qemuDomainBlockCommit(virDomainPtr dom,
     if (!(device = qemuAliasDiskDriveFromDisk(disk)))
         goto endjob;
 
-    if (!disk->src->path) {
+    if (virStorageSourceIsEmpty(disk->src)) {
         virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
                        _("disk %s has no source file to be committed"),
                        disk->dst);