qcow2: Write full header on image creation

When creating a qcow2 image, we didn't necessarily call
qcow2_update_header(), but could end up with the basic header that
qcow2_create2() created manually. One thing that this basic header
lacks is the feature table. Let's make sure that it's always present.

This requires a few updates to test cases as well.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
diff --git a/tests/qemu-iotests/031.out b/tests/qemu-iotests/031.out
index f065404..7f5050b 100644
--- a/tests/qemu-iotests/031.out
+++ b/tests/qemu-iotests/031.out
@@ -116,6 +116,11 @@
 header_length             104
 
 Header extension:
+magic                     0x6803f857
+length                    144
+data                      <binary>
+
+Header extension:
 magic                     0x12345678
 length                    31
 data                      'This is a test header extension'