vl.c: Remove compile time limit on number of serial ports

vl.c: Remove compile time limit on number of serial ports

Instead of having a fixed sized global serial_hds[] array,
use a local dynamically reallocated one, so we don't have
a compile time limit on how many serial ports a system has.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180420145249.32435-13-peter.maydell@linaro.org