fw_cfg: Use void *, size_t instead of uint8_t *, uint32_t for blobs

Many callers pass size_t, which gets silently truncated to uint32_t.
Harmless, because all practical sizes are well below 4GiB.  Clean it
up anyway.  Size overflow now fails assertions.

Bonus: saves a whole bunch of silly casts.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
