Implement support for the slice of type 'storage' which allows to set
the offset and size which modifies where qemu should look for the start
of the format container inside the image.

Since slicing is done using the 'raw' driver we need to add another
layer into the blockdev tree if there's any non-raw image format driver
used to access the data.

This patch adds the blockdev integration and setup of the image data so
that we can use the slices for any backing image.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>