xen bits broke x32 build

[Michael Tokarev]

After bringing in xen guest support, qemu fails to build on x32:

target/i386/kvm/xen-emu.c:876:5: note: in expansion of macro ‘qemu_build_assert’
  876 |     qemu_build_assert(sizeof(struct vcpu_info) == 64);
      |     ^~~~~~~~~~~~~~~~~

This one should be easy to fix, but I wonder if there are other issues
with x32 exists..

Thanks,

/mjt