How to generate correctly the "set root" with grub2

[Renato _]

Hey Everyone.

am working to set up with packer "Bionic ubuntu images" and our KVM-Hypervisor 
has some refinement. I am looking a way to generate the grub.cfg with specific 
params on. And I don't know if it's possible.

This is what is generated by default when I boot the host at 1st time :
set root='hd0,msdos1'
root=/dev/vda1

and this is what I would like to the end :
set root=(hd0)
root=/dev/vda

I set "/boot/grub/device.map" to :
(hd0)   /dev/vda
(hd1)   /dev/vdb

my guess, I should play with the "boot/grub/custom.cfg" file? Do you have some 
examples?

Also, I realized that enabling GRUB_DISABLE_LINUX_UUID by true,  that still 
generate UUID on the file.  Is is that expected?

Many Thanks.
t.

boot_grub_grub.txt
Description: boot_grub_grub.txt

etc_default_grub.txt
Description: etc_default_grub.txt