init layer is read/write layer and not read only layer. Following commit
introduced new graph driver method CreateReadWrite.
ef5bfad Adding readOnly parameter to graphdriver Create method
So far only windows seem to be differentiating between above two methods.
Making this change to make sure -init layer calls right method so that
we don't have surprises in future.
Windows does not need init layer. This patch also gets rid of creation of
init layer on windows.
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>