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>

devices/src/legacy/fw_cfg.rs

@@ -909,6 +909,32 @@ mod unit_tests {
         }
     }
 
+    #[test]
+    fn test_string_item() {
+        let gm = GuestMemoryAtomic::new(
+            GuestMemoryMmap::from_ranges(&[(GuestAddress(0), RAM_64BIT_START.0 as usize)]).unwrap(),
+        );
+
+        let mut fw_cfg = FwCfg::new(gm);
+
+        // Simulate OVMF X-PciMmio64Mb string item for GPU CC passthrough
+        let item = FwCfgItem {
+            name: "opt/ovmf/X-PciMmio64Mb".to_owned(),
+            content: FwCfgContent::Bytes("262144".as_bytes().to_vec()),
+        };
+        fw_cfg.add_item(item).unwrap();
+
+        let expected = b"262144";
+        let mut data = vec![0u8];
+
+        // Select the first file item (FW_CFG_FILE_FIRST = 0x20)
+        fw_cfg.write(0, SELECTOR_OFFSET, &[FW_CFG_FILE_FIRST as u8, 0]);
+        for &byte in expected.iter() {
+            fw_cfg.read(0, DATA_OFFSET, &mut data);
+            assert_eq!(data[0], byte);
+        }
+    }
+
     #[test]
     fn test_dma() {
         let code = [