qcow2: make qcow2_cache_put() a void function

This function never receives an invalid table pointer, so we can make
it void and remove all the error checking code.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
