man 5 passwd states following;

[...]
password  the encrypted user password, an asterisk (*), or the letter 'x'.  (See pwconv(8) for an explanation of 'x'.)
[...]

so handle both situation

diff --git a/config.planetlab/vserver.post b/config.planetlab/vserver.post
index def0615..7d518f4 100644 (file)
--- a/config.planetlab/vserver.post
+++ b/config.planetlab/vserver.post
@@ -10,7 +10,7 @@ fi
 
 # reset root password to be empty
 echo "Resetting root password in vserver-reference image"
-sed -i -e "s/root:\*/root:/" ${vdir}/etc/passwd
+sed -i -e "s/root:[\*|x]/root:/" ${vdir}/etc/passwd
 
 # modify the sudoers file to remove the 'requiretty' attribute
 echo "Removing requiretty from default /etc/sudoers file"