disk-image-create should exit once it determines there's not enough RAM (currently it still proceeds)

disk-image-create should exit once it determines there's note enough RAM. Currently it throws a message that there isn't enough memory (fair enough), but still goes ahead and tries to create in /tmp/... (which is also on 'tmpfs' for Fedora)

Env:

  $ git clone https://github.com/stackforge/diskimage-builder.git
  $ git log | head -1
  commit ad0fe876df8c7a8d45712163f143e48516b077b3

-> Try to create a guest in an a VM (with less than 4G mem): -- I tested this on a fresh F19 VM

  $ disk-image-create -o fedora-test fedora -a i386 2>&1 | tee /var/tmp/dib-log-3.txt
Building elements: base fedora
If prompted for sudo, install sudoers.d/img-build-sudoers into /etc/sudoers.d and restart the build.
Expanded element dependencies to: dib-run-parts dracut-network base fedora cache-url
Not enough RAM to use tmpfs for build. (4049868 < 4G)
Building in /tmp/image.AqpUhtDJ
/tmp/image.AqpUhtDJ/hooks/root.d/10-fedora-cloud-image:

Fetching Base Image
  % Total % Received % Xferd Average Speed Time Time Time Current
                                 Dload Upload Total Spent Left Speed
  0 0 0 0 0 0 0 0 --:--:-- 0:00:01 --:--:-- 0
Server copy has not changed. Using locally cached http://mattdm.fedorapeople.org/cloud-images//Fedora18-Cloud-i386-latest.qcow2
Repacking base image as tarball.
Working in /tmp/tmp.nS7yxI2TKd
mount: special device /dev/mapper/loop0p2 does not exist
loop deleted : /dev/loop0
touch: cannot touch '/tmp/image.AqpUhtDJ/mnt/etc/resolv.conf': No such file or directory
umount: /tmp/image.AqpUhtDJ/mnt/sys: not found
umount: /tmp/image.AqpUhtDJ/mnt/proc: not found
umount: /tmp/image.AqpUhtDJ/mnt/dev: not found
umount: /tmp/image.AqpUhtDJ/mnt/tmp/in_target.d: not found

umount: /tmp/image.AqpUhtDJ/mnt: not mounted
umount: /tmp/image.AqpUhtDJ: not mounted

-> /tmp is tmpfs

[root@localhost diskimage-builder]# df -hT
Filesystem Type Size Used Avail Use% Mounted on
/dev/mapper/VolGroup00-LogVol00 ext4 23G 1.1G 21G 5% /
devtmpfs devtmpfs 491M 0 491M 0% /dev
tmpfs tmpfs 498M 0 498M 0% /dev/shm
tmpfs tmpfs 498M 300K 498M 1% /run
tmpfs tmpfs 498M 0 498M 0% /sys/fs/cgroup
tmpfs tmpfs 498M 12K 498M 1% /tmp
/dev/vda2 ext4 190M 61M 120M 34% /boot

-> Fedora 19:

  $ cat /etc/fedora-release
  Fedora release 19 (Schrödinger’s Cat)