target/i386: Fix broken build with WHPX enabled

The WHPX build is broken since commit 12e9493df9 which removed the
"hw/boards.h" where MachineState is declared:

  $ ./configure \
     --enable-hax --enable-whpx

  $ make x86_64-softmmu/all
  [...]
    CC      x86_64-softmmu/target/i386/whpx-all.o
  target/i386/whpx-all.c: In function 'whpx_accel_init':
  target/i386/whpx-all.c:1378:25: error: dereferencing pointer to
  incomplete type 'MachineState' {aka 'struct MachineState'}
       whpx->mem_quota = ms->ram_size;
                           ^~
  make[1]: *** [rules.mak:69: target/i386/whpx-all.o] Error 1
    CC      x86_64-softmmu/trace/generated-helpers.o
  make[1]: Target 'all' not remade because of errors.
  make: *** [Makefile:471: x86_64-softmmu/all] Error 2

Restore this header, partially reverting commit 12e9493df9.

Fixes: 12e9493df9
Reported-by: Ilias Maratos <i.maratos@gmail.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20190920113329.16787-2-philmd@redhat.com>

target/i386/whpx-all.c

@@ -18,6 +18,7 @@
 #include "sysemu/cpus.h"
 #include "sysemu/runstate.h"
 #include "qemu/main-loop.h"
+#include "hw/boards.h"
 #include "qemu/error-report.h"
 #include "qapi/error.h"
 #include "migration/blocker.h"