KVM: PIT: control word is write-only

PIT control word (address 0x43) is write-only, reads are undefined.

Cc: [e-mail]
Signed-off-by: Marcelo Tosatti <[e-mail]>

arch/x86/kvm/i8254.c

···
 		return -EOPNOTSUPP;
 
 	addr &= KVM_PIT_CHANNEL_MASK;
+	if (addr == 3)
+		return 0;
+
 	s = &pit_state->channels[addr];
 
 	mutex_lock(&pit_state->lock);