kvm tools: Fixes for disk image module

Fixes include:
 - Error handling
 - Cleanup
 - Standard init/uninit

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
diff --git a/disk/qcow.c b/disk/qcow.c
index 712f811..23f11f2 100644
--- a/disk/qcow.c
+++ b/disk/qcow.c
@@ -1334,7 +1334,7 @@
 	else
 		disk_image = disk_image__new(fd, h->size, &qcow_disk_ops, DISK_IMAGE_REGULAR);
 
-	if (!disk_image)
+	if (IS_ERR_OR_NULL(disk_image))
 		goto free_refcount_table;
 
 	disk_image->async = 0;