KVM: nVMX: Check for pending posted interrupts when looking for nested events

KVM: nVMX: Check for pending posted interrupts when looking for nested events

Check for pending (and notified!) posted interrupts when checking if L2
has a pending wake event, as fully posted/notified virtual interrupt is a
valid wake event for HLT.

Note that KVM must check vmx->nested.pi_pending to avoid prematurely
waking L2, e.g. even if KVM sees a non-zero PID.PIR and PID.0N=1, the
virtual interrupt won't actually be recognized until a notification IRQ is
received by the vCPU or the vCPU does (nested) VM-Enter.

Fixes: 26844fee6ade ("KVM: x86: never write to memory from kvm_vcpu_check_block()")
Cc: stable@vger.kernel.org
Cc: Maxim Levitsky <mlevitsk@redhat.com>
Reported-by: Jim Mattson <jmattson@google.com>
Closes: https://lore.kernel.org/all/20231207010302.2240506-1-jmattson@google.com
Link: https://lore.kernel.org/r/20240607172609.3205077-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>