x86/kvm: Update the comment about asynchronous page fault in exc_page_fault()

KVM was switched to interrupt-based mechanism for 'page ready' event
delivery in Linux-5.8 (see commit 2635b5c4 ("KVM: x86: interrupt based
APF 'page ready' event delivery")) and #PF (ab)use for 'page ready' event
delivery was removed. Linux guest switched to this new mechanism
exclusively in 5.9 (see commit b1d40575

 ("KVM: x86: Switch KVM guest to
using interrupts for page ready APF delivery")) so it is not possible to
get #PF for a 'page ready' event even when the guest is running on top
of an older KVM (APF mechanism won't be enabled). Update the comment in
exc_page_fault() to reflect the new reality.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20201002154313.1505327-1-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>