KVM: x86: Always set mp_state to RUNNABLE on wakeup from HLT

KVM: x86: Always set mp_state to RUNNABLE on wakeup from HLT

When emulating HLT and a wake event is already pending, explicitly mark
the vCPU RUNNABLE (via kvm_set_mp_state()) instead of assuming the vCPU is
already in the appropriate state.  Barring a KVM bug, it should be
impossible for the vCPU to be in a non-RUNNABLE state, but there is no
advantage to relying on that to hold true, and ensuring the vCPU is made
RUNNABLE avoids non-deterministic behavior with respect to pv_unhalted.

E.g. if the vCPU is not already RUNNABLE, then depending on when
pv_unhalted is set, KVM could either leave the vCPU in the non-RUNNABLE
state (set before __kvm_emulate_halt()), or transition the vCPU to HALTED
and then RUNNABLE (pv_unhalted set after the kvm_vcpu_has_events() check).

Link: https://lore.kernel.org/r/20250224174156.2362059-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>