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 <[email protected]>
Signed-off-by: Blue Swirl <[email protected]>
diff --git a/trace-events b/trace-events
index cf76a11..7de9106 100644
--- a/trace-events
+++ b/trace-events
@@ -172,7 +172,7 @@
 fw_cfg_select(void *s, uint16_t key, int ret) "%p key %d = %d"
 fw_cfg_read(void *s, uint8_t ret) "%p = %d"
 fw_cfg_add_file_dupe(void *s, char *name) "%p %s"
-fw_cfg_add_file(void *s, int index, char *name, uint32_t len) "%p #%d: %s (%d bytes)"
+fw_cfg_add_file(void *s, int index, char *name, size_t len) "%p #%d: %s (%zd bytes)"
 
 # hw/hd-geometry.c
 hd_geometry_lchs_guess(void *bs, int cyls, int heads, int secs) "bs %p LCHS %d %d %d"