fix(host): add check mem dev is null on guest setup (#22847)

Author: wanyaoqi

pkg/hostman/guestman/qemu-kvm.go

@@ -3015,7 +3015,7 @@ func (s *SKVMGuestInstance) startHotPlugVcpus(vcpuSet []int) error {
 
 func (s *SKVMGuestInstance) hotPlugCpus() error {
 	var vcpuSet = make([]int, 0)
-	if len(s.Desc.MemDesc.Mem.Mems) > 0 {
+	if s.Desc.MemDesc.Mem != nil && len(s.Desc.MemDesc.Mem.Mems) > 0 {
 		for i := range s.Desc.CpuNumaPin {
 			for j := range s.Desc.CpuNumaPin[i].VcpuPin {
 				vcpuSet = append(vcpuSet, s.Desc.CpuNumaPin[i].VcpuPin[j].Vcpu)