Detect image format for qemu-nbd --format

qemu-nbd will error when the image format is raw unless there is an
explicit --format raw argument provided.

This change adds format detection for raw and qcow2 using qemu-img
info. Any other image type will depend on qemu-nbd auto-detection.

Blueprint: whole-disk-default
Change-Id: I628abc4cb8c46c3b2e5bfd3cd16bb6d5cc95fabe

scripts/tripleo-mount-image

@@ -92,7 +92,14 @@ remove_device () {
 mount_image() {
     set -x
 
-    qemu-nbd --connect $NBD_DEVICE $IMAGE_FILE
+    if qemu-img info --output json $IMAGE_FILE |grep '"format": "raw"' ; then
+        image_format='--format raw'
+    elif qemu-img info --output json $IMAGE_FILE |grep '"format": "qcow2"' ; then
+        image_format='--format qcow2'
+    else
+        image_format=''
+    fi
+    qemu-nbd $image_format --connect $NBD_DEVICE $IMAGE_FILE
 
     # search for the vg volume group, this is automatic in some environments
     vgscan