util/qemu-config: fix regression of qmp_query_command_line_options

Commit 49d2e648 (machine: remove qemu_machine_opts global list)
made machine options specific to machine sub-type, leaving
the qemu_machine_opts desc array empty. Sadly this is the place
qmp_query_command_line_options is looking for supported options.

As a fix for for 2.3 the machine_qemu_opts (the generic ones)
are restored only for qemu-config scope.
We need to find a better fix for 2.4.

Reported-by: Tony Krowiak <akrowiak@linux.vnet.ibm.com>
Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Message-Id: <1427906841-1576-1-git-send-email-marcel@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>