devices: Add fw_cfg device

Here we add the fw_cfg device as a legacy device to the device manager.
It is guarded behind a fw_cfg flag in vmm at creation of the
DeviceManager. In this cl we implement the fw_cfg device with one
function (signature).

Signed-off-by: Alex Orozco <alexorozco@google.com>

Cargo.toml

@@ -64,6 +64,7 @@ wait-timeout = { workspace = true }
 dbus_api = ["vmm/dbus_api", "zbus"]
 default = ["io_uring", "kvm"]
 dhat-heap = ["dhat", "vmm/dhat-heap"]       # For heap profiling
+fw_cfg = ["vmm/fw_cfg"]
 guest_debug = ["vmm/guest_debug"]
 igvm = ["mshv", "vmm/igvm"]
 io_uring = ["vmm/io_uring"]