KVM: Add dedicated arch hook for querying if vCPU was preempted in-kernel

Plumb in a dedicated hook for querying whether or not a vCPU was preempted
in-kernel.  Unlike literally every other architecture, x86's VMX can check
if a vCPU is in kernel context if and only if the vCPU is loaded on the
current pCPU.

x86's kvm_arch_vcpu_in_kernel() works around the limitation by querying
kvm_get_running_vcpu() and redirecting to vcpu->arch.preempted_in_kernel
as needed.  But that's unnecessary, confusing, and fragile, e.g. x86 has
had at least one bug where KVM incorrectly used a stale
preempted_in_kernel.

No functional change intended.

Reviewed-by: Yuan Yao <yuan.yao@intel.com>
Link: https://lore.kernel.org/r/20240110003938.490206-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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