qemu-iotests: Be more flexible with image creation options

qemu-iotests already filters out image creation options that may be
present or not in order to get the same output in both cases. However,
often it only considers the default value of the option. Cover all valid
values instead so that ./check -o name=value can be used successfull for
all of them.

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