Added support for custom qemu-img format options

When kiwi calls qemu-img to convert the image into the format
the user has specified in the XML description it might be
required to pass on additional format options like the compat
mode for the qcow2 format. This can now be done like the
following example shows:

<type ... formatoptions="compat=0.10"/>
Merge pull request #419 from openSUSE/review_141211_image_format_options

Added support for custom qemu-img format options