dracut: fix checking for "Root filesystem" label, improve udev sync

Don't try to dereference "Root filesytem" partlabel symlink, unless it's
really present (not only directory for it).

Also, use udevadm settle for waiting for /dev/xvda, instead of naive
wait sleep loop.

dracut/full-dmroot/qubes_cow_setup.sh

@@ -53,12 +53,12 @@ fi
 modprobe xenblk || modprobe xen-blkfront || warn "Qubes: Cannot load Xen Block Frontend..."
 
 log_begin "Waiting for /dev/xvda* devices..."
-while ! [ -e /dev/xvda ]; do sleep 0.1; done
+udevadm settle --exit-if-exists=/dev/xvda
 log_end
 
 # prefer partition if exists
 if [ -b /dev/xvda1 ]; then
-    if [ -d /dev/disk/by-partlabel ]; then
+    if [ -e "/dev/disk/by-partlabel/Root\\x20filesystem" ]; then
         ROOT_DEV=$(basename $(readlink "/dev/disk/by-partlabel/Root\\x20filesystem"))
     else
         ROOT_DEV=xvda3