Revert "sandbox: Change md command to use map_physmem"

This reverts commit 355a8357.

The original commit broke long standing assumption that md commands work
on effective addresses.  This normally isn't an issue for most systems
that map 1:1, however on systems with a 36-bit address map it breaks.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>