xen/manage: Use orderly_reboot() to reboot

Currently when the toolstack issues a reboot, it gets translated into a
call to ctrl_alt_del(). But tying reboot to ctrl-alt-del means rebooting
may fail if e.g. the user has masked the ctrl-alt-del.target under
systemd.

A previous attempt to fix this issue made a change that sets the
kernel.ctrl-alt-del sysctl to 1 before ctrl_alt_del() is called.
However, this doesn't give userspace the opportunity to block rebooting
or even do any cleanup or syncing.

Instead, call orderly_reboot() which will call the "reboot" command,
giving userspace the opportunity to block it or perform the usual reboot
process while being independent of the ctrl-alt-del behaviour. It also
matches what happens in the shutdown case.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20220627142822.3612106-1-ross.lagerwall@citrix.com
Signed-off-by: Juergen Gross <jgross@suse.com>