Mounting /dev/root on /root failed: no such device

[Mohammad Hedayati]

I'm using Ubuntu 11.04 and I managed to compile Xen 4.0.1 on it using
this tutorial:
http://blog.davidwolinsky.com/2010/12/installing-xen-401-and-testing-on.html
Everything went fine, but using this menuentry:
menuentry 'Ubuntu GNU/Linux, with Xen 4 and Linux 2.6.32.41' --class
ubuntu --class gnu-linux --class gnu --class os --class xen {
        insmod ext2
        set root='(hd0,2)'
        echo            'Loading Xen 4 ...'
        multiboot       (hd0,2)/boot/xen.gz console=vga
        echo            'Loading Linux 2.6.32.41 ...'
        module          (hd0,2)/boot/vmlinuz-2.6.32.41 root=/dev/sda2
nopat ro console=tty0
        echo            'Loading initial ramdisk ...'
        module          (hd0,2)/boot/initrd.img-2.6.32.41
}
but while booting I get:
mount: Mounting /dev/root on /root failed: no such device
mount: Mounting /dev on /root/dev/ failed: no such file or directory
mount: Mounting /sys on /root/sys failed: no such file or directory
....
BusyBox v1.17.1 ....