qemu-iotests: Speed up make check-block

Using /tmp, which is usually mounted as tmpfs, the quick group can be
quicker.

On my laptop (Lenovo T430s with Fedora 20), this reduces the time from
50s to 30s.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
