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

(cherry picked from commit 636d377264f51e3dd33bd7f33ebf03e2e148d40d)