How do I get Android-x86 (32-bit or 64-bit) non-UEFI to boot in VirtualBox?
When you try to install Android x86 on pc HDD, try to provide a boot partition through Create/modify partition and make it bootable. Followed by that create a new partition as Linux. write both the partition and after that install the android x86 on the linux partition (formatting as ext4). 

It will work, after allocating a bootable partition. I tried and it worked.