Probe for capacity in virStorageBackendUpdateVolTargetInfo

Instead of just looking at the output of fstat, call
virStorageFileGetMetadata to get the full capacity from
image headers.

Note that the capacity is probed unconditionally. The updateCapacity
bool parameter is ignored and will be removed in the following commit.

src/storage/storage_backend.c

@@ -1514,6 +1514,9 @@ virStorageBackendUpdateVolTargetInfo(virStorageSourcePtr target,
 {
     int ret, fd = -1;
     struct stat sb;
+    virStorageSourcePtr meta = NULL;
+    char *buf = NULL;
+    ssize_t len = VIR_STORAGE_MAX_HEADER;
 
     if ((ret = virStorageBackendVolOpen(target->path, &sb, openflags)) < 0)
         goto cleanup;
@@ -1523,14 +1526,41 @@ virStorageBackendUpdateVolTargetInfo(virStorageSourcePtr target,
                                                       updateCapacity)) < 0)
         goto cleanup;
 
+    if (target->type == VIR_STORAGE_VOL_FILE &&
+        target->format != VIR_STORAGE_FILE_NONE) {
+        if (S_ISDIR(sb.st_mode)) {
+            ret = 0;
+            goto cleanup;
+        }
+
+        if (lseek(fd, 0, SEEK_SET) == (off_t)-1) {
+            virReportSystemError(errno, _("cannot seek to start of '%s'"), target->path);
+            goto cleanup;
+        }
+
+        if ((len = virFileReadHeaderFD(fd, len, &buf)) < 0) {
+            virReportSystemError(errno, _("cannot read header '%s'"), target->path);
+            goto cleanup;
+        }
+
+        if (!(meta = virStorageFileGetMetadataFromBuf(target->path, buf, len, target->format,
+                                                      NULL))) {
+            goto cleanup;
+        }
+
+        if (meta->capacity)
+            target->capacity = meta->capacity;
+    }
+
     if (withBlockVolFormat) {
         if ((ret = virStorageBackendDetectBlockVolFormatFD(target, fd)) < 0)
             goto cleanup;
     }
 
  cleanup:
+    virStorageSourceFree(meta);
     VIR_FORCE_CLOSE(fd);
-
+    VIR_FREE(buf);
     return ret;
 }