qemu-iotests: Handle cache mode option in 091 We should allow testing this on tmpfs. Any cache setting in iotests should try to obey $CACHEMODE. The cache mode is still "none" by default but overridable Signed-off-by: Fam Zheng <famz@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>