emulators/virtualbox-ose{,-nox11}: add vboxinit start/stop script for VMs which is controlled from phpvirtualbox

Set the "Startup Mode" to "Automatic" for the virtual machine in
phpvirtualbox to automatically start the virtual machine during OS boot.

This script also stops virtual machines during reboot even if
vboxinit_enable="YES" is not present in /etc/rc.conf.

phpvirtualbox uses the "Web Service" (WEBSERVICE) to configure and
manage virtual machines, so we install the script only when the
WEBSERVICE option is enabled.

PR: 280062
(cherry picked from commit d0deb35b6952e0d73c111c6534462dcbd5d77774)