Clone with VirtualBox

I want to test severeal configuration like kde with gentoo or other test configuration.That's why, I create a common gentoo virtual machine and I clone it.
You can clone your virtual machine like that :

VBoxManage clonevdi ${HOME}/.VirtualBox/HardDisks/Gentoo_common.vdi ${HOME}/.VirtualBox/HardDisks/Gentoo_kde.vdi

Then, Clic add and after when you add you hard drive, choose your vdi file.

EXT3-fs couldn't mount because unsupported option No file system could mount root

I decide to update my Gentoo Virtual machine on my VirtualBox.
I configured my /etc/fstab like that :

/dev/sda1   /boot        ext4    defaults,noatime     1 2
/dev/sda2   none         swap    sw                   0 0
/dev/sda3   /            ext4    noatime              0 1
/dev/cdrom  /mnt/cdrom   auto    noauto,user          0 0

proc        /proc        proc    defaults             0 0
shm         /dev/shm     tmpfs   nodev,nosuid,noexec  0 0

I also apply the EXT4 filesystem with mkfs.ext4

When I ran my Virtual machine, I have a kernel panic with the following message : EXT3-fs couldn't mount because unsupported option No file system could mount root
In fact, I forgot to add EXT4 support to my kernel. See this link to add EXT4 support on your kernel : EXT4_kernel