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/dm/vioapic.c

@@ -401,7 +401,7 @@ vioapic_indirect_write(struct acrn_vioapic *vioapic, uint32_t addr,
 				((last.full & IOAPIC_RTE_INTMASK) == 0UL)) {
 			/* VM enable intr */
 			/* NOTE: only support max 256 pin */
-			ptdev_intx_pin_remap(vioapic->vm,
+			(void)ptdev_intx_pin_remap(vioapic->vm,
 					(uint8_t)pin, PTDEV_VPIN_IOAPIC);
 		}
 	}
@@ -516,7 +516,7 @@ vioapic_init(struct acrn_vm *vm)
 
 	vioapic_reset(vm_ioapic(vm));
 
-	register_mmio_emulation_handler(vm,
+	(void)register_mmio_emulation_handler(vm,
 			vioapic_mmio_access_handler,
 			(uint64_t)VIOAPIC_BASE,
 			(uint64_t)VIOAPIC_BASE + VIOAPIC_SIZE,