qemu-iotests: Add "-c <cache-mode>" option

The option sets cache mode used in the tests. "-nocache" is changed to
an alias to "-c none", and internally passes "-t none" to qemu-io.

Python scripts will make use of option this in the next commit.

Signed-off-by: Fam Zheng <famz@redhat.com>
qemu-iotests: Honour cache mode in iotests.py

This will allow overriding cache mode from the "-c mode" option.

Signed-off-by: Fam Zheng <famz@redhat.com>
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>
qemu-iotests: Change default cache mode to "writeback"

So that the tests can run faster.

Signed-off-by: Fam Zheng <famz@redhat.com>
qemu-iotests: Clean up spaces in usage output

Whitespace changes to align columns.

Signed-off-by: Fam Zheng <famz@redhat.com>
qemu-iotests: Split qcow2 only cases in 048

Format "raw" doesn't always work on certain file systems (e.g. tmpfs).
Use qcow2 to make the allocation status explicit and split into a new
case.

Signed-off-by: Fam Zheng <famz@redhat.com>