qemu-iotests: Add _default_cache_mode and _supported_cache_modes

This replaces _unsupported_qemu_io_options and check for support of
current cache mode, and allow to provide a default if user didn't
specify.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>