raw-win32.c: Fix incorrect handling behaviour of small block files

It is a valid case that the read data's size is smaller than the
requested size since there could be files that are smaller than
the minimum block size (For ex. when a VMDK disk descriptor file)

Signed-off-by: Tal Kain <tal.kain@ravellosystems.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
