Grub boot loader installation fails after installing ubuntu server 16.04 on virtual machine

The host is Ubuntu 16.04 the virtual machine is created using VirtualBox 5.1. I have used the following command to create virtual disk which is using raw partitions from disk /dev/sdc (that how it appears on the host and has 4 partitions - 1 primary for the host and 1 swap for the host, 1 logical (sdc7) and 1 swap (sdc8) for the virtual disk).

sudo VBoxManage intealcommands createrawvmdk -filename "/home/hd4/VirtualBox VMs/ws1/ws1_disk.vmdk" -rawdisk /dev/sdc -partitions 7,8 -relative

The real disk is SSD (if have any reason to know that). I attached the virtual disk as SATA device and the ISO for Ubuntu server 16.04 as IDE controller/device. All of the things are going fine except the installation of the Grub Boot Loader, sadly. After that I have tried manually to install the boot loader on:

/dev/sda
/dev/sda7
/dev/sda8

As you seen on the virtual machine the disk is sda, normally.

Basically I have prepared the sdc7 and sdc8 partitions to have the desirable sizes and they should be ready to use. During installation I marked sda7 to be mounter to / as ext4 and sda8 as swap.

I have peddling experience with boot loaders and have no idea what to try next. Any suggestions and rationalizations are welcome.