qemu-iotests: Add -o and make v3 the default for qcow2

This adds an -o option to qemu-iotests, which is an option string that
is passed through to qemu-img create -o... This allows testing different
subformat with a command like './check -qcow2 -o compat=0.10'.

For qcow2, if no compat option is specified, compat=1.1 is the new
default.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
