KVM: selftests: Allow skipping the KVM_RUN sanity check in rseq_test

The rseq test's migration worker delays 1-10 us, assuming that one KVM_RUN
iteration only takes a few microseconds.  But if the CPU low power wakeup
latency is large enough, for example, hundreds or even thousands of
microseconds for deep C-state exit latencies on x86 server CPUs, it may
happen that the target CPU is unable to wakeup and run the vCPU before the
migration worker starts to migrate the vCPU thread to the _next_ CPU.

If the system workload is light, most CPUs could be at a certain low
power state, which may result in less successful migrations and fail the
migration/KVM_RUN ratio sanity check.  But this is not supposed to be
deemed a test failure.

Add a command line option to skip the sanity check, along with a comment
and a verbose assert message to try to help the user resolve the potential
source of failures without having to resort to disabling the check.

Co-developed-by: Dongsheng Zhang <dongsheng.x.zhang@intel.com>
Signed-off-by: Dongsheng Zhang <dongsheng.x.zhang@intel.com>
Signed-off-by: Zide Chen <zide.chen@intel.com>
Link: https://lore.kernel.org/r/20240502213936.27619-1-zide.chen@intel.com
[sean: massage changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>