block: use /var/tmp instead of /tmp for -snapshot

If TMPDIR is not specified, the default was to use /tmp for the working
copy of the block devices.  Update this to /var/tmp instead, so systems
using tmp-on-tmpfs don't end up inadvertently using RAM for the block
device.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>