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>