ui: move qemu_input_linux_to_qcode()

Move from input-linux.c to input-keymap.c and export it,
so the function is available elsewhere too.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20170726152918.11995-3-kraxel@redhat.com