softmmu/ioport.c: allocate MemoryRegionPortioList ports on the heap

softmmu/ioport.c: allocate MemoryRegionPortioList ports on the heap

In order to facilitate a conversion of MemoryRegionPortioList to a QOM object
move the allocation of MemoryRegionPortioList ports to the heap instead of
using a variable-length member at the end of the MemoryRegionPortioList
structure.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230419151652.362717-2-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>