drivers/vga: Fix VGAConsole::write()

Spotted by Clang:

../../drivers/vga.hh:15:18: error: 'VGAConsole::write' hides overloaded virtual function
      [-Werror,-Woverloaded-virtual]
    virtual void write(const char *str);
                 ^
../../drivers/console.hh:16:18: note: hidden overloaded virtual function 'Console::write'
      declared here: different number of parameters (2 vs 1)
    virtual void write(const char *str, size_t len) = 0;
                 ^

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>