qapi: Extract ACPI commands to 'acpi.json'

Only qemu-system-FOO and qemu-storage-daemon provide QMP
monitors, therefore such declarations and definitions are
irrelevant for user-mode emulation.

Extracting the ACPI commands to their own schema reduces the size of
the qapi-misc* headers generated, and pulls less QAPI-generated code
into user-mode.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Igor Mammedov <imammedo@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200913195348.1064154-8-philmd@redhat.com>
[Commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>