hv: fix '(void) missing for discarded return value'

MISRA-C requires that the function call in which the returned
value is discarded shall be clearly indicated using (void).

This patch fixes the violations related to the following
function calls.
- instr_check_gva
- vlapic_set_local_intr
- prepare_vm
- enter_s3
- emulate_instruction
- ptdev_intx_pin_remap
- register_mmio_emulation_handler

v1 -> v2:
 * discard the return value of enter_s3

Tracked-On: #861
Signed-off-by: Shiqing Gao <shiqing.gao@intel.com>

hypervisor/arch/x86/guest/instr_emul.c

@@ -2293,7 +2293,10 @@ int decode_instruction(struct acrn_vcpu *vcpu)
 			return retval;
 		}
 	} else {
-		instr_check_gva(vcpu, emul_ctxt, cpu_mode);
+		retval = instr_check_gva(vcpu, emul_ctxt, cpu_mode);
+		if (retval < 0) {
+			return retval;
+		}
 	}
 
 	return (int)(emul_ctxt->vie.opsize);