goldfish_nand: add buffer sizes to vmsd for load/savevm
goldfish_nand: fix loadvm to restore the NAND's buffers properly

Each NAND device has a name buffer and a scrach buffer. Using QEMU's
buit-in save/load handlers, the system assumes the pointers to
those buffers are already pointing to valid allocated buffers
before running loadvm. Even though the buffer sizes are part of
the serialized data, there is no way to access them.

This commit enforces the maximum size of the buffer contents and
allocates the buffers at the max size before the loadvm handler runs.
If the data is too large for either buffer QEMU will abort.
The data can only be too large for the buffer if
the NAND devices' parameters are altered. There is no way to do this
aside from recompiling. These will need to be altered for ext4 support,
but that will result in a *smaller* required buffer size.