block/vpc: Make vpc_checksum() take void *

Some of the next commits will checksum structs.  Change vpc_checksum()
to take void * instead of uint8_t, to save us pointless casts to
uint8_t *.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201217162003.1102738-5-armbru@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/vpc.c

@@ -172,8 +172,9 @@ static QemuOptsList vpc_runtime_opts = {
 
 static QemuOptsList vpc_create_opts;
 
-static uint32_t vpc_checksum(uint8_t *buf, size_t size)
+static uint32_t vpc_checksum(void *p, size_t size)
 {
+    uint8_t *buf = p;
     uint32_t res = 0;
     int i;