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

block.c

@@ -547,8 +547,9 @@ int get_tmp_filename(char *filename, int size)
     int fd;
     const char *tmpdir;
     tmpdir = getenv("TMPDIR");
-    if (!tmpdir)
-        tmpdir = "/tmp";
+    if (!tmpdir) {
+        tmpdir = "/var/tmp";
+    }
     if (snprintf(filename, size, "%s/vl.XXXXXX", tmpdir) >= size) {
         return -EOVERFLOW;
     }