KVM: x86: Add a return code to inject_pending_event

No functional change intended. At present, 'r' will always be -EBUSY
on a control transfer to the 'out' label.

Signed-off-by: Jim Mattson <jmattson@google.com>
Message-Id: <20210604172611.281819-5-jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>