vmm, devices: Add fw_cfg string item support

QEMU supports passing inline string values to the guest via fw_cfg
(-fw_cfg name=...,string=...). Cloud Hypervisor previously only
supported file-backed fw_cfg items. This adds the 'string' option
so users can pass values like OVMF's X-PciMmio64Mb without creating
a temporary file on the host.

Each fw_cfg item now accepts exactly one of 'file' or 'string'.
The FwCfgInvalidItem invariant is validated in PayloadConfig::validate()
(via FwCfgConfig::validate()), covering both CLI and JSON API paths.
The populate_fw_cfg match arm uses unreachable!() since validation
guarantees the invariant holds at that point.

CLI syntax:
  --fw-cfg-config items=[name=opt/ovmf/X-PciMmio64Mb,string=262144]

Signed-off-by: Keith Adler <kadler@cloudflare.com>

cloud-hypervisor/tests/integration.rs

@@ -11538,4 +11538,45 @@ mod fw_cfg {
 
         handle_child_output(r, &output);
     }
+
+    #[test]
+    #[cfg_attr(feature = "mshv", ignore = "See #7434")]
+    fn test_fw_cfg_string() {
+        let disk_config = UbuntuDiskConfig::new(JAMMY_IMAGE_NAME.to_string());
+        let guest = Guest::new(Box::new(disk_config));
+        let mut cmd = GuestCommand::new(&guest);
+
+        let kernel_path = direct_kernel_boot_path();
+        let cmd_line = DIRECT_KERNEL_BOOT_CMDLINE;
+
+        cmd.args(["--cpus", "boot=4"])
+            .default_memory()
+            .args(["--kernel", kernel_path.to_str().unwrap()])
+            .args(["--cmdline", cmd_line])
+            .default_disks()
+            .default_net()
+            .args([
+                "--fw-cfg-config",
+                "initramfs=off,items=[name=opt/org.test/test-string,string=hello-from-vmm]",
+            ])
+            .capture_output();
+
+        let mut child = cmd.spawn().unwrap();
+
+        let r = std::panic::catch_unwind(|| {
+            guest.wait_vm_boot().unwrap();
+            thread::sleep(std::time::Duration::new(3, 0));
+            let result = guest
+                .ssh_command(
+                    "sudo cat /sys/firmware/qemu_fw_cfg/by_name/opt/org.test/test-string/raw",
+                )
+                .unwrap();
+            assert_eq!(result, "hello-from-vmm");
+        });
+
+        kill_child(&mut child);
+        let output = child.wait_with_output().unwrap();
+
+        handle_child_output(r, &output);
+    }
 }