KVM: x86/xen: Fix runstate updates to be atomic when preempting vCPU

There are circumstances whem kvm_xen_update_runstate_guest() should not
sleep because it ends up being called from __schedule() when the vCPU
is preempted:

[  222.830825]  kvm_xen_update_runstate_guest+0x24/0x100
[  222.830878]  kvm_arch_vcpu_put+0x14c/0x200
[  222.830920]  kvm_sched_out+0x30/0x40
[  222.830960]  __schedule+0x55c/0x9f0

To handle this, make it use the same trick as __kvm_xen_has_interrupt(),
of using the hva from the gfn_to_hva_cache directly. Then it can use
pagefault_disable() around the accesses and just bail out if the page
is absent (which is unlikely).

I almost switched to using a gfn_to_pfn_cache here and bailing out if
kvm_map_gfn() fails, like kvm_steal_time_set_preempted() does — but on
closer inspection it looks like kvm_map_gfn() will *always* fail in
atomic context for a page in IOMEM, which means it will silently fail
to make the update every single time for such guests, AFAICT. So I
didn't do it that way after all. And will probably fix that one too.

Cc: stable@vger.kernel.org
Fixes: 30b5c851

 ("KVM: x86/xen: Add support for vCPU runstate information")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <b17a93e5ff4561e57b1238e3e7ccd0b613eb827e.camel@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>