QEMU insists on using /var/tmp instead of /tmp

Description of problem

On a host, our sysadmins have decided for whatever reason that /var/tmp is not a thing that normal users can write to (and perhaps that's dumb, but it is what it is and would be a challenging non-technical problem to solve). Whenever QEMU detects the temporary directory is /tmp, it changes it to /var/tmp without a mechanism to change it (see 69fbfff9).

I'm sure in the general case this is fine, but can you add an environment variable or a ./configure option to make this location configurable? I really would like to write to /tmp.