Convert the SPARC CPU from cpu_load/save functions to VMStateDescription.
We preserve migration compatibility with the previous version
(required for SPARC32 but not necessarily for SPARC64).
Backports commit df32c8d436d4eb3f40b00647ca0df2bbc7f6bf6f from qemu
The callers (most of them in target-foo/cpu.c) to this function all
have the cpu pointer handy. Just pass it to avoid an ENV_GET_CPU() from
core code (in exec.c).
Backports commit 4bad9e392e788a218967167a38ce2ae7a32a6231 from qemu
