qapi: Restrict query-vm-generation-id command to machine code

qapi: Restrict query-vm-generation-id command to machine code

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

Restricting the query-vm-generation-id command to machine.json pulls
less QAPI-generated code into user-mode.

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: <20200913195348.1064154-5-philmd@redhat.com>
[Commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>