initramfs: add support for variable partitions layout

Try to find root filesystem by partition label (not filesystem label!).
If that fails, default to 3rd partition according to (new) default
layout.

Fixes QubesOS/qubes-issues#3173

dracut/full-dmroot/qubes_cow_setup.sh

@@ -56,9 +56,13 @@ log_begin "Waiting for /dev/xvda* devices..."
 while ! [ -e /dev/xvda ]; do sleep 0.1; done
 log_end
 
-# prefer first partition if exists
+# prefer partition if exists
 if [ -b /dev/xvda1 ]; then
-    ROOT_DEV=xvda1
+    if [ -d /dev/disk/by-partlabel ]; then
+        ROOT_DEV=$(basename $(readlink "/dev/disk/by-partlabel/Root\\x20filesystem"))
+    else
+        ROOT_DEV=xvda3
+    fi
 else
     ROOT_DEV=xvda
 fi

dracut/simple/init.sh

@@ -20,9 +20,18 @@ die() {
 echo "Waiting for /dev/xvda* devices..."
 while ! [ -e /dev/xvda ]; do sleep 0.1; done
 
-# prefer first partition if exists
+# prefer partition if exists
 if [ -b /dev/xvda1 ]; then
-    ROOT_DEV=xvda1
+    if [ -d /dev/disk/by-partlabel ]; then
+        ROOT_DEV=$(basename $(readlink "/dev/disk/by-partlabel/Root\\x20filesystem"))
+    else
+        ROOT_DEV=$(grep -l "PARTNAME=Root filesystem" /sys/block/xvda/xvda*/uevent |\
+            grep -o "xvda[0-9]")
+    fi
+    if [ -z "$ROOT_DEV" ]; then
+        # fallback to third partition
+        ROOT_DEV=xvda3
+    fi
 else
     ROOT_DEV=xvda
 fi