When no mapping is requested, it is pointless to create
alias regions.
Only create them when multiple mappings are requested to
simplify the memory layout. The flatview is not changed.

For example using 'qemu-system-sh4 -M r2d -S -monitor stdio',

* before:

  (qemu) info mtree
  address-space: memory
    0000000000000000-ffffffffffffffff (prio 0, i/o): system
      0000000000000000-0000000000ffffff (prio 0, i/o): pflash
        0000000000000000-0000000000ffffff (prio 0, romd): alias pflash-alias @r2d.flash 0000000000000000-0000000000ffffff
      0000000004000000-000000000400003f (prio 0, i/o): r2d-fpga
      000000000c000000-000000000fffffff (prio 0, ram): r2d.sdram
  (qemu) info mtree -f
  FlatView #0
   AS "memory", root: system
   AS "cpu-memory-0", root: system
   Root memory region: system
    0000000000000000-0000000000ffffff (prio 0, romd): r2d.flash
    0000000004000000-000000000400003f (prio 0, i/o): r2d-fpga
    000000000c000000-000000000fffffff (prio 0, ram): r2d.sdram

* after:

  (qemu) info mtree
  address-space: memory
    0000000000000000-ffffffffffffffff (prio 0, i/o): system
      0000000000000000-0000000000ffffff (prio 0, romd): r2d.flash
      0000000004000000-000000000400003f (prio 0, i/o): r2d-fpga
      000000000c000000-000000000fffffff (prio 0, ram): r2d.sdram
  (qemu) info mtree -f
  FlatView #0
   AS "memory", root: system
   AS "cpu-memory-0", root: system
   Root memory region: system
    0000000000000000-0000000000ffffff (prio 0, romd): r2d.flash
    0000000004000000-000000000400003f (prio 0, i/o): r2d-fpga
    000000000c000000-000000000fffffff (prio 0, ram): r2d.sdram

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210325120921.858993-3-f4bug@amsat.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>