release: vmimage: setup the default BE properly

All of the BE datasets need to be set canmount=noauto so that creating
a new BE and switching to it can actually work. With the current setup,
the zfs rc script will mount the default BE over whichever new BE is
activated once it runs.

Reported by: andrew
Reviewed by: andrew, re (cperciva), imp, markj
Differential Revision: https://reviews.freebsd.org/D48017