KVM: x86: Don't clear async #PF queue when CR0.PG is disabled (e.g. on #SMI)

Fix an interaction between SMM and PV asynchronous #PFs where an #SMI can
cause KVM to drop an async #PF ready event, and thus result in guest tasks
becoming permanently stuck due to the task that encountered the #PF never
being resumed.  Specifically, don't clear the completion queue when paging
is disabled, and re-check for completed async #PFs if/when paging is
enabled.

Prior to commit 2635b5c4a0e4 ("KVM: x86: interrupt based APF 'page ready'
event delivery"), flushing the APF queue without notifying the guest of
completed APF requests when paging is disabled was "necessary", in that
delivering a #PF to the guest when paging is disabled would likely confuse
and/or crash the guest.  And presumably the original async #PF development
assumed that a guest would only disable paging when there was no intent to
ever re-enable paging.

That assumption fails in several scenarios, most visibly on an emulated
SMI, as entering SMM always disables CR0.PG (i.e. initially runs with
paging disabled).  When the SMM handler eventually executes RSM, the
interrupted paging-enabled is restored, and the async #PF event is lost.

Similarly, invoking firmware, e.g. via EFI runtime calls, might require a
transition through paging modes and thus also disable paging with valid
entries in the competion queue.

To avoid dropping completion events, drop the "clear" entirely, and handle
paging-enable transitions in the same way KVM already handles APIC
enable/disable events: if a vCPU's APIC is disabled, APF completion events
are not kept pending and not injected while APIC is disabled.  Once a
vCPU's APIC is re-enabled, KVM raises KVM_REQ_APF_READY so that the vCPU
recognizes any pending pending #APF ready events.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20251015033258.50974-4-mlevitsk@redhat.com
[sean: rework changelog to call out #PF injection, drop "real mode"
       references, expand the code comment]
Signed-off-by: Sean Christopherson <seanjc@google.com>

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