KVM: selftests: Add basic stress test for save+restore and #PF handling

Add a basic stress test for handling #PFs in a guest while the host is
doing save+restore cycles. The guest periodically accesses non-present
memory causing a #PF, and the #PF handler walks the page tables and
updates the PTE to be present, like a proper #PF handler.

After every access (and #PF), the guest triggers a sync and the test
performs save+restore of the VM. This is not very meaningful as
save+restore are performed after the access and #PF handling complete,
but following changes will change that.

Assisted-by: Gemini:gemini-3.1-pro
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
Link: https://patch.msgid.link/20260728174232.2423257-11-yosry@kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>

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