TMPDIR is not usable for snapshot-blockdevs, if not root
Description of problem:
for using static disk-content we're using `snapshot`-flag for certain disks and set `TMPDIR` to a VM-specific path.

when started as root, all is ok.

when started as non-root, `getenv(TMPDIR)` in function `get_tmp_filename()` in file `block.c` return `NULL`, because glibc handles `TMPDIR` as `UNSECURE_ENVVAR` (glibc-src: `sysdeps/generic/unsecvars.h`)

well, we could compile qemu by ourself, but then we might miss important updates, so maybe this can be solved in main-source?

possible solutions: 
- additionally look at another var like `QEMU_TMPDIR`, if `getenv("TMPDIR")` results in `NULL`
- add a global option to qemu like `--tmpdir=...`
- add a device-specific option like `snapshotdir=...`