util/qemu-config: fix regression of qmp_query_command_line_options

util/qemu-config: fix regression of qmp_query_command_line_options

Commit 49d2e64 (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 <[email protected]>
Signed-off-by: Marcel Apfelbaum <[email protected]>
Message-Id: <1427906841[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>