qapi/machine: Make @dump-skeys command generic

qapi/machine: Make @dump-skeys command generic

Reduce misc-target.json by one target specific command.

Error message is returned for machines not implementing
TYPE_DUMP_SKEYS_INTERFACE:

  $ qemu-system-aarch64 -M virt -S -qmp stdio
  {"QMP": {"version": {"qemu": {"micro": 50, "major": 9}}, "capabilities": ["oob"]}}
  { "execute": "qmp_capabilities" }
  {"return": {}}
  { "execute": "dump-skeys", "arguments": { "filename": "/tmp/foo" }  }
  {"error": {"class": "GenericError", "desc": "Storage keys information not available for this architecture"}}

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20250310151414.11550-5-philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>