packer: Do not write during quick test

In order to being able play with generated image many times avoid
needless writes there by using qemu -snapshot switch.

packer/README.md

@@ -83,6 +83,16 @@ In order to test one image (after unzip), you can run:
     bash util/quick-test packer-centos72 "-display sdl"
     
 
+Watch out. quick-test will NOT write data into the image, so after exiting,
+the image will be reverted back to its original state.
+
+In order to have your work saved using quick test, please use information from
+qemu man page:
+
+       -snapshot
+           Write to temporary files instead of disk image files. In this case, the raw disk image you use is not written back. You can however force the write back by
+           pressing C-a s.
+
 Extra
 -----
 

packer/util/quick-test

 #/bin/bash -e
 # Quick command to test the VM image with output file.
 
-/usr/bin/qemu-system-x86_64 $2 -drive file=$1,if=virtio,cache=writeback,discard=ignore -boot once=d -m 512M -name test -machine type=pc,accel=kvm -netdev user,id=user.0,hostfwd=tcp::3213-:22 -device virtio-net,netdev=user.0 -vnc 0.0.0.0:47
+echo "Changes will not be written"
+echo "To force write please use:"
+echo "man qemu: You can however force the write back by pressing C-a s."
+/usr/bin/qemu-system-x86_64 $2 -snapshot -drive file=$1,if=virtio,cache=writeback,discard=ignore -boot once=d -m 512M -name test -machine type=pc,accel=kvm -netdev user,id=user.0,hostfwd=tcp::3213-:22 -device virtio-net,netdev=user.0 -vnc 0.0.0.0:47