KVM: VMX: Handle KVM-induced preemption timer exits in fastpath for L2

KVM: VMX: Handle KVM-induced preemption timer exits in fastpath for L2

Eat VMX treemption timer exits in the fastpath regardless of whether L1 or
L2 is active.  The VM-Exit is 100% KVM-induced, i.e. there is nothing
directly related to the exit that KVM needs to do on behalf of the guest,
thus there is no reason to wait until the slow path to do nothing.

Opportunistically add comments explaining why preemption timer exits for
emulating the guest's APIC timer need to go down the slow path.

Link: https://lore.kernel.org/r/20240110012705.506918-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>