sandbox: mmc: Start off with a zeroed file

When running multiple tests the mmc emulator calls malloc() to obtain the
memory for its disk image. Since the memory is not cleared, it is possible
that it happens to contain a partition table.

The dm_test_part() test (for one) relies on mmc0 being empty on startup.
Zero the memory to ensure that it is.

Signed-off-by: Simon Glass <sjg@chromium.org>