src: add vhost-user-blk backend

Create a vhost-user-blk backend using vhost-user-backend and following
the conventions established by the existing vhost-user-net
implementation.

This backend is based on https://github.com/slp/vhost-user-backend,
but a bit simplified, making it closer to the original implementation
in Firecracker. The main features missing are EVENT_IDX, support for
asynchronous I/O and multiqueue, but it's still fully functional and
provides a good starting point for evolving it into a more complete
implementation.

Signed-off-by: Sergio Lopez <slp@redhat.com>

Cargo.toml

@@ -12,6 +12,7 @@ libc = "0.2.65"
 log = { version = "0.4.8", features = ["std"] }
 net_gen = { path = "net_gen" }
 net_util = { path = "net_util" }
+qcow = { path = "qcow" }
 vhost_user_backend = { path = "vhost_user_backend"}
 virtio-bindings = "0.1.0"
 vmm = { path = "vmm" }