hw/sd.c: fix migration of dynamically allocated buffer "buf"

hw/sd.c: fix migration of dynamically allocated buffer "buf"

VMSTATE_BUFFER_UNSAFE should be used for buffers inlined in device state, not
for buffers allocated dynamically. Change to VMSTATE_BUFFER_POINTER_UNSAFE macro,
which will do migration right.

Signed-off-by: Igor Mitsyanko <i.mitsyanko@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Michael Walle <michael@walle.cc>
Message-id: 1362923278-4080-3-git-send-email-i.mitsyanko@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>