Do:

vzctl set 101 --userpasswd root:password

where 101 is the container id

Notes from the manual:

–userpasswd user:password
Sets password for the given user in a container, creating the user if it does not exists. Note that this option is not saved in configura‐
tion file at all (so –save flag is useless), it is applied to the container (by modifying its /etc/passwd and /etc/shadow files).

In case container root filesystem is not mounted, it is automatically mounted, then all the appropriate file changes are applied, then it is
unmounted.

Note that container should be created before using this option.