Comment # 7 on bug 931787 from Diego Ercolani

Solved!
This is what I did:
1. create a logical volume to receive the btrfs root filesystem created
formatting the device
2. create all the subvolume structure in the new ntrfs volume
3. copied all the files with a "cp -ax" from every subvolume
4. mount -o bind all the /dev /sys /*** to the new rootfs
5. mkinitrd
6. grub2-install /dev/sda; grub2-install /dev/sdb
7. grub2-mkconfig -o /boot/grub2/grub.cfg
8. reboot

All is working now in the new partition

So my conclusion is: there is something weird in the original btrfs partition
that boot process cannot understand.

Since I resolved the issue on my own, do you think I can trash the old
partition or you need me to try to understand what has gone wrong? (but please
point out what to do)