virtio: Remove unneeded memcpy

virtio: Remove unneeded memcpy

Report from valgrind:

==19521== Source and destination overlap in memcpy(0x31d38938, 0x31d38938, 64)
==19521==    at 0x4A0A343: memcpy@@GLIBC_2.14 (in
/usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==19521==    by 0x42774E: virtio_blk_device_init (virtio-blk.c:686)
==19521==    by 0x46EE9E: virtio_device_init (virtio.c:1158)
==19521==    by 0x25405E: device_realize (qdev.c:178)
==19521==    by 0x2559B5: device_set_realized (qdev.c:699)
==19521==    by 0x3A819B: property_set_bool (object.c:1315)
==19521==    by 0x3A6CE0: object_property_set (object.c:803)

Valgrind is right: blk == &s->blks, so it is a memcpy of 64 byte with
source == destination which can be removed.

Reported-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>