I think what you want is
xfce4-session-logout (online manpage).
Excerpt from the manpage (reformatted, filtered):
The xfce4-session-logout command allows you to programmatically logout
from your Xfce session. It requests the session manager to display the
logout confirmation screen, or, if given one of the command-line
options below, causes the session manager to take the requested action
--logout Log out without displaying the logout dialog.
--halt Halt without displaing the logout dialog.
--reboot Reboot without displaying the logout dialog.
--suspend Suspend without displaying the logout dialog.
--hibernate Hibernate without displaying the logout dialog.
--fast Do a fast shutdown. This instructs the session manager not to
save the session, but instead to quit everything quickly.
So to shut down, use
and to reboot, use
If you just want to get the dialogue where you can pick an action manually, run it without arguments: