msix: implement pba write (but read-only)

qpci_msix_pending() writes on pba region, causing qemu to SEGV:

  Program received signal SIGSEGV, Segmentation fault.
  [Switching to Thread 0x7ffff7fba8c0 (LWP 25882)]
  0x0000000000000000 in ?? ()
  (gdb) bt
  #0  0x0000000000000000 in  ()
  #1  0x00005555556556c5 in memory_region_oldmmio_write_accessor (mr=0x5555579f3f80, addr=0, value=0x7fffffffbf68, size=4, shift=0, mask=4294967295, attrs=...) at /home/elmarco/src/qemu/memory.c:434
  #2  0x00005555556558e1 in access_with_adjusted_size (addr=0, value=0x7fffffffbf68, size=4, access_size_min=1, access_size_max=4, access=0x55555565563e <memory_region_oldmmio_write_accessor>, mr=0x5555579f3f80, attrs=...) at /home/elmarco/src/qemu/memory.c:506
  #3  0x00005555556581eb in memory_region_dispatch_write (mr=0x5555579f3f80, addr=0, data=0, size=4, attrs=...) at /home/elmarco/src/qemu/memory.c:1176
  #4  0x000055555560b6f9 in address_space_rw (as=0x555555eff4e0 <address_space_memory>, addr=3759147008, attrs=..., buf=0x7fffffffc1b0 "", len=4, is_write=true) at /home/elmarco/src/qemu/exec.c:2439
  #5  0x000055555560baa2 in cpu_physical_memory_rw (addr=3759147008, buf=0x7fffffffc1b0 "", len=4, is_write=1) at /home/elmarco/src/qemu/exec.c:2534
  #6  0x000055555564c005 in cpu_physical_memory_write (addr=3759147008, buf=0x7fffffffc1b0, len=4) at /home/elmarco/src/qemu/include/exec/cpu-common.h:80
  #7  0x000055555564cd9c in qtest_process_command (chr=0x55555642b890, words=0x5555578de4b0) at /home/elmarco/src/qemu/qtest.c:378
  #8  0x000055555564db77 in qtest_process_inbuf (chr=0x55555642b890, inbuf=0x55555641b340) at /home/elmarco/src/qemu/qtest.c:569
  #9  0x000055555564dc07 in qtest_read (opaque=0x55555642b890, buf=0x7fffffffc2e0 "writel 0xe0100800 0x0\n", size=22) at /home/elmarco/src/qemu/qtest.c:581
  #10 0x000055555574ce3e in qemu_chr_be_write (s=0x55555642b890, buf=0x7fffffffc2e0 "writel 0xe0100800 0x0\n", len=22) at qemu-char.c:306
  #11 0x0000555555751263 in tcp_chr_read (chan=0x55555642bcf0, cond=G_IO_IN, opaque=0x55555642b890) at qemu-char.c:2876
  #12 0x00007ffff64c9a8a in g_main_context_dispatch (context=0x55555641c400) at gmain.c:3122

(without this patch, this can be reproduced with the ivshmem qtest)

Implement an empty mmio write to avoid the crash.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
