KVM: x86 emulator: fix handling for unemulated instructions

If an instruction is present in the decode tables but not in the execution
switch, it will be emulated as a NOP.  An example is IRET (0xcf).

Fix by adding default: labels to the execution switches.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/emulate.c

@@ -3028,6 +3028,8 @@ special_insn:
 		if (c->modrm_reg == 5)
 			goto jump_far;
 		goto grp45;
+	default:
+		goto cannot_emulate;
 	}
 
 writeback:
@@ -3353,6 +3355,8 @@ twobyte_insn:
 		if (rc != X86EMUL_CONTINUE)
 			goto done;
 		break;
+	default:
+		goto cannot_emulate;
 	}
 	goto writeback;