ui/curses: Move arrays to .heap to save 74KiB of .bss

We only need these arrays when using the curses display.
Move them from the .bss to the .heap (sizes reported on
x86_64 host: screen[] is 64KiB, vga_to_curses 7KiB).

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>