It is actually rather easy.
Brendan Kidwell's Web site gives you a good intro to his script to start your VM at boot. One of the great advantages of this script is that it does not shutdown your VM if you power off your host, but rather it allows to configure things so that your VM is hybernated. That means that, upon host reboot, the VM will pick up wherever it had left off.
The script can be found here.
In order to complete your request, all you have to do is to introduce the call to mount the shared folder in the sub-routine do_wait_for_online, immediately before the line that states:
echo Ready
and to start the service not with start, but with start-wait. The difference between the two is that the second one will wait for the VM to come online, which is the earliest moment in which you can mount a shared folder via sshfs.