qemu-iotests: Test cache mode option inheritance

This is doing a more complete test on setting cache modes both while
opening an image (i.e. in a -drive command line) and in reopen
situations. It checks that reopen can specify options for child nodes
and that cache modes are correctly inherited from parent nodes where
they are not specified.

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