release/oci.conf: add missing vm_extra_pre_umount() steps

vm.subr's default vm_extra_pre_umount removes /qemu and
/etc/resolv.conf. When vm_extra_pre_umount is overridden these steps
need to be performed in the cloud-specific conf file.

PR: 271602
Reviewed by: dch, lwhsu
Event: Kitchener-Waterloo Hackathon 202305
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40257