option: Remove shadowing opt decl from qemu_opt_print()

opt was declared as a separate local inside the last loop,
shadowing the local at the top of the function.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20171005190725.18712-1-dgilbert@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
