KVM: selftests: Trigger save+restore randomly in the #PF stress test

Instead of an explicit GUEST_SYNC() after each access+#PF, run another
thread that keeps sending SIGUSR to the vCPU thread, essentially
triggering exits to userspace and save+restore on random points in guest
execution. This makes the test a lot more meaningful as it opens the
door to exercising race conditions between #PF handling in the guest
and save+restore in the host.

The signals are ignored using SIG_IGN outside of __vcpu_run() to avoid
interrupting other ioctls/sysctls performed by the test.

Assisted-by: Gemini:gemini-3.1-pro
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
Link: https://patch.msgid.link/20260728174232.2423257-12-yosry@kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>

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