xen: expose support for poweroff/reboot/suspend on xenbus

Some toolstacks won't attempt the signal power actions on xenbus unless the VM
explicitly exposes support for them. FreeBSD supports all power actions, hence
signal on xenbus such support by setting the nodes to the value of "1".

Sponsored by: Cloud Software Group
Reviewed by: markj
Differential review: https://reviews.freebsd.org/D46859