Move KVM keymap from GenerateRuntime to ExecuteRuntime

Per issue 243, "side-effects" are GenerateRuntime are bad as they
execute only on the initial node of the instance. By moving the
write-out of the keymap file to ExecuteRuntime, it will be done both
at start and at migrate time.

Furthermore, we update the docstring of GenerateKVMRuntime to explain
this, and add a fixme related to the spice per-interface binding.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>