KVM: x86: Fold kvm_get_apic_interrupt() into kvm_cpu_get_interrupt()

Fold kvm_get_apic_interrupt() into kvm_cpu_get_interrupt() now that nVMX
essentially open codes kvm_get_apic_interrupt() in order to correctly
emulate nested posted interrupts.

Opportunistically stop exporting kvm_cpu_get_interrupt(), as the
aforementioned nVMX flow was the only user in vendor code.

Link: https://lore.kernel.org/r/20240906043413.1049633-6-seanjc@google.com


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