bootstrapvz: Tweak to firstboot.sh

Works around a bug/oddity in boostrapvz where the virtual disk
is initialized with a gpt that is actually smaller than the
disk itself, being rather the sum of the sizes of the partitions
(which confuses things as we leave much of the space
unpartitionned).

Change-Id: Ifbd9e5e8eb39c09fe12a3807fac8ef1ffcb8e894
Merge "bootstrapvz: Tweak to firstboot.sh" into production