KVM: x86: Add a return code to kvm_apic_accept_events

No functional change intended. At present, the only negative value
returned by kvm_check_nested_events is -EBUSY.

Signed-off-by: Jim Mattson <jmattson@google.com>
Message-Id: <20210604172611.281819-6-jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>