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

(cherry picked from commit ae60012e46)

release/tools/oci.conf

@@ -90,5 +90,10 @@ EOF
 
 	touch ${DESTDIR}/firstboot
 
+	if ! [ -z "${QEMUSTATIC}" ]; then
+		rm -f ${DESTDIR}/${EMULATOR}
+	fi
+	rm -f ${DESTDIR}/etc/resolv.conf
+
 	return 0
 }