KVM: x86: WARN if a vCPU gets a valid wakeup that KVM can't yet inject

WARN if a blocking vCPU is awakened by a valid wake event that KVM can't
inject, e.g. because KVM needs to complete a nested VM-enter, or needs to
re-inject an exception.  For the nested VM-Enter case, KVM is supposed to
clear "nested_run_pending" if L1 puts L2 into HLT, i.e. entering HLT
"completes" the nested VM-Enter.  And for already-injected exceptions, it
should be impossible for the vCPU to be in a blocking state if a VM-Exit
occurred while an exception was being vectored.

Link: https://lore.kernel.org/r/20240607172609.3205077-7-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>