hw/i386/vmport: Allow QTest use without crashing

hw/i386/vmport: Allow QTest use without crashing

Trying libFuzzer on the vmport device, we get:

  AddressSanitizer:DEADLYSIGNAL
  =================================================================
  ==29476==ERROR: AddressSanitizer: SEGV on unknown address 0x000000008840 (pc 0x56448bec4d79 bp 0x7ffeec9741b0 sp 0x7ffeec9740e0 T0)
  ==29476==The signal is caused by a READ memory access.
    #0 0x56448bec4d78 in vmport_ioport_read (qemu-fuzz-i386+0x1260d78)
    #1 0x56448bb5f175 in memory_region_read_accessor (qemu-fuzz-i386+0xefb175)
    #2 0x56448bb30c13 in access_with_adjusted_size (qemu-fuzz-i386+0xeccc13)
    #3 0x56448bb2ea27 in memory_region_dispatch_read1 (qemu-fuzz-i386+0xecaa27)
    #4 0x56448bb2e443 in memory_region_dispatch_read (qemu-fuzz-i386+0xeca443)
    #5 0x56448b961ab1 in flatview_read_continue (qemu-fuzz-i386+0xcfdab1)
    #6 0x56448b96336d in flatview_read (qemu-fuzz-i386+0xcff36d)
    #7 0x56448b962ec4 in address_space_read_full (qemu-fuzz-i386+0xcfeec4)

This is easily reproducible using:

  $ echo inb 0x5658 | qemu-system-i386 -M isapc,accel=qtest -qtest stdio
  [I 1589796572.009763] OPENED
  [R +0.008069] inb 0x5658
  Segmentation fault (core dumped)

  $ coredumpctl gdb -q
  Program terminated with signal SIGSEGV, Segmentation fault.
  #0  0x00005605b54d0f21 in vmport_ioport_read (opaque=0x5605b7531ce0, addr=0, size=4) at hw/i386/vmport.c:77
  77          eax = env->regs[R_EAX];
  (gdb) p cpu
  $1 = (X86CPU *) 0x0
  (gdb) bt
  #0  0x00005605b54d0f21 in vmport_ioport_read (opaque=0x5605b7531ce0, addr=0, size=4) at hw/i386/vmport.c:77
  #1  0x00005605b53db114 in memory_region_read_accessor (mr=0x5605b7531d80, addr=0, value=0x7ffc9d261a30, size=4, shift=0, mask=4294967295, attrs=...) at memory.c:434
  #2  0x00005605b53db5d4 in access_with_adjusted_size (addr=0, value=0x7ffc9d261a30, size=1, access_size_min=4, access_size_max=4, access_fn=
      0x5605b53db0d2 <memory_region_read_accessor>, mr=0x5605b7531d80, attrs=...) at memory.c:544
  #3  0x00005605b53de156 in memory_region_dispatch_read1 (mr=0x5605b7531d80, addr=0, pval=0x7ffc9d261a30, size=1, attrs=...) at memory.c:1396
  #4  0x00005605b53de228 in memory_region_dispatch_read (mr=0x5605b7531d80, addr=0, pval=0x7ffc9d261a30, op=MO_8, attrs=...) at memory.c:1424
  #5  0x00005605b537c80a in flatview_read_continue (fv=0x5605b7650290, addr=22104, attrs=..., ptr=0x7ffc9d261b4b, len=1, addr1=0, l=1, mr=0x5605b7531d80) at exec.c:3200
  #6  0x00005605b537c95d in flatview_read (fv=0x5605b7650290, addr=22104, attrs=..., buf=0x7ffc9d261b4b, len=1) at exec.c:3239
  #7  0x00005605b537c9e6 in address_space_read_full (as=0x5605b5f74ac0 <address_space_io>, addr=22104, attrs=..., buf=0x7ffc9d261b4b, len=1) at exec.c:3252
  #8  0x00005605b53d5a5d in address_space_read (len=1, buf=0x7ffc9d261b4b, attrs=..., addr=22104, as=0x5605b5f74ac0 <address_space_io>) at include/exec/memory.h:2401
  #9  0x00005605b53d5a5d in cpu_inb (addr=22104) at ioport.c:88

X86CPU is NULL because QTest accelerator does not use CPU.
Fix by returning default values when QTest accelerator is used.

Reported-by: Clang AddressSanitizer
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>