storage: Fixing missing 'backingStore' tag from volume XML dumps.

After commit a693fdba 'vol-dumpxml' missed the ability to show backingStore
information. This commit adds a volume type for files that fixes this
problem.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1529663Signed-off-by: NJulio Faracco <jcfaracco@gmail.com>

src/storage/storage_util.c

@@ -1957,6 +1957,8 @@ virStorageBackendUpdateVolTargetInfoFD(virStorageSourcePtr target,
     target->timestamps->ctime = get_stat_ctime(sb);
     target->timestamps->mtime = get_stat_mtime(sb);
 
+    target->type = VIR_STORAGE_TYPE_FILE;
+
     VIR_FREE(target->perms->label);
 
 #if WITH_SELINUX