block: implement bdrv_co_is_allocated() boundary cases Cases beyond the end of the disk image are only implemented for block drivers that do not provide .bdrv_co_is_allocated(). It's worth making these cases generic so that block drivers that do implement .bdrv_co_is_allocated() also get them for free. Suggested-by: Mark Wu <wudxw@linux.vnet.ibm.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>