The QERR_ macros are leftovers from the days of "rich" error objects.
We've been trying to reduce their remaining use.

qmp_query_vm_generation_id() in stubs/vmgenid.c is the last user of
QERR_UNSUPPORTED outside qga/.  Unlike the stubs we just dropped, it
is actually reachable, namely when CONFIG_ACPI_VMGENID is off.  It
always fails like

    (qemu) info vm-generation-id
    Error: this feature or command is not currently supported

Turns out the real qmp_query_vm_generation_id() doesn't actually
depend on CONFIG_ACPI_VMGENID, and fails safely when it's off.  Move
it to hw/core/machine-qmp-cmds.c, and drop the stub.  The error
message becomes

    Error: VM Generation ID device not found

Feels like an improvement to me.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230207075115.1525-8-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>