To disable the timesync for a Virtualbox guest, run the following command on the host
vboxmanage setextradata <VMname> "VBoxInternal/Devices/VMMDev/0/Config/GetHostTimeDisabled" "1"
and to re-enable
vboxmanage setextradata <VMname> "VBoxInternal/Devices/VMMDev/0/Config/GetHostTimeDisabled" "0"