I've setup a drbd replication between two machines and used a drbd block device as storage for a kvm machine. Everything is running well. However I'm in doubt if this setup is ok to use. From what I've read so far on the internet, people tend to use drbd->ocfs2->qcow2_file as storage for their virtual machines.
We use DRBD in production with KVM, works like a charm. We also use it without a filesystem, so the setup is very similar.
No reason to use qcow2 unless you're taking snapshots, the raw format is faster.
DRBD is not something I'd trust in production. If you have a centralised storage, NFS should be enough, or if you prefer block storage - use LVM and switch a host access rights with lvchange