I messed up the bootloader by accident on a standard Centos 6.3 install because I turned the /dev/vda1 boot partition into an mdadm raid 1. This was all done correctly aside from one point I didn't realize was an issue metadata=00.90 is the only thing that will allow you to boot (otherwise grub won't work and you won't boot).
So the next step is rescue mode from a CD right? The problem you will find is that grub does not detect your hard drives, this is I believe is because of virtio or it just isn't accustomed to /dev/vd* naming conventions.
The grub install fix for KVM
The main difference is that we are assigning hd0 manually and telling it to be /dev/vda.
device (hd0) /dev/vda
Repeat the above for any other disks you want to install grub on eg.
device (hd1) /dev/vdb
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