hv: hypercall: VM management fix "Procedure has more than one exit point"

IEC 61508,ISO 26262 standards highly recommend single-exit rule.

Reduce the count of the "return entries".
Fix the violations which is comply with the cases list below:
1.Function has 2 return entries.
2.The first return entry is used to return the error code of
checking variable whether is valid.

Fix the violations in "if else" format.
V1->V2:
    update the git comment to describe why comply with the
single-exit rule.

V2->V3:
    update the git comment title to give a scope declaration of this
patch.

Tracked-On: #861
Signed-off-by: Huihuang Shi <huihuang.shi@intel.com>
Reviewed-by: Li, Fei1 <fei1.li@intel.com>
Acked-by: Eddie Dong <eddie.dong@intel.com>

Author: Shawnshh

hypervisor/common/hypercall.c

@@ -157,10 +157,11 @@ int32_t hcall_destroy_vm(uint16_t vmid)
 	struct acrn_vm *target_vm = get_vm_from_vmid(vmid);
 
 	if (target_vm == NULL) {
-		return -1;
+		ret = -1;
+	} else {
+		ret = shutdown_vm(target_vm);
 	}
 
-	ret = shutdown_vm(target_vm);
 	return ret;
 }
 
@@ -181,9 +182,8 @@ int32_t hcall_start_vm(uint16_t vmid)
 	struct acrn_vm *target_vm = get_vm_from_vmid(vmid);
 
 	if (target_vm == NULL) {
-		return -1;
-	}
-	if (target_vm->sw.io_shared_page == NULL) {
+		ret = -1;
+	} else if (target_vm->sw.io_shared_page == NULL) {
 		ret = -1;
 	} else {
 		ret = start_vm(target_vm);
@@ -206,14 +206,16 @@ int32_t hcall_start_vm(uint16_t vmid)
 int32_t hcall_pause_vm(uint16_t vmid)
 {
 	struct acrn_vm *target_vm = get_vm_from_vmid(vmid);
+	int32_t ret;
 
 	if (target_vm == NULL) {
-		return -1;
+	        ret = -1;
+	} else {
+		pause_vm(target_vm);
+		ret = 0;
 	}
 
-	pause_vm(target_vm);
-
-	return 0;
+	return ret;
 }
 
 /**
@@ -273,12 +275,14 @@ int32_t hcall_create_vcpu(struct acrn_vm *vm, uint16_t vmid, uint64_t param)
 int32_t hcall_reset_vm(uint16_t vmid)
 {
 	struct acrn_vm *target_vm = get_vm_from_vmid(vmid);
+	int32_t ret;
 
 	if ((target_vm == NULL) || is_vm0(target_vm)) {
-		return -1;
+	        ret = -1;
+	} else {
+	        ret = reset_vm(target_vm);
 	}
-
-	return reset_vm(target_vm);
+	return ret;
 }
 
 /**