release: increase VMSIZE for Oracle images

While OK for 14.x, both 13.x and 15.0 are failing in snapshot
builds.

Reported by: cperciva
Sponsored by: SkunkWerks, GmbH

Reviewed by: emaste
Approved by: cperciva
Differential Revision: https://reviews.freebsd.org/D47285

(cherry picked from commit 89311e6f987ebb1a968eee6fe594b11bfb91977a)