feat(migtd): route warn/error/fatal logs to COM1 serial port

MigTD debugging has historically required ad-hoc TDVMCALL.IO writes
inserted by hand, because failure paths often did not call any of
the `log::*` macros and there was no guarantee that fatal exits
reached the host console.

td-logger is already initialised with `LevelFilter::Trace` and, with
the `tdx` feature, writes each byte to COM1 (0x3F8) via TDVMCALL.IO,
which QEMU forwards through `-serial mon:stdio`. This change makes
that serial path the canonical debugging channel:

  * Add `release_max_level_info` to the `log` dependency so release
    builds compile out `debug!`/`trace!` while keeping `info!`,
    `warn!` and `error!` on the serial line. No VMM changes, no
    shared buffer and no extra TDVMCALL are required.
  * Emit `log::error!` from `panic_with_guest_crash_reg_report` with
    the error code, message and caller `file:line` so every fatal
    exit is visible on the serial port even when the surrounding
    code forgot to log.

Signed-off-by: Michal Tarnacki <michal.tarnacki@intel.com>
Co-authored-by: GitHub Copilot <noreply@github.com>

Author: MichalTarnacki

src/migtd/src/driver/vmcall_raw.rs

@@ -16,8 +16,6 @@ pub fn vmcall_raw_device_init() {
 
 #[track_caller]
 pub fn panic_with_guest_crash_reg_report(errorcode: u64, msg: &[u8]) -> ! {
-    #[cfg(not(feature = "vmcall-raw"))]
-    let _ = errorcode;
     let location = core::panic::Location::caller();
     let file = location.file();
     let line = location.line();
@@ -27,6 +25,16 @@ pub fn panic_with_guest_crash_reg_report(errorcode: u64, msg: &[u8]) -> ! {
         " non-UTF8 message".to_string()
     };
 
+    // Always surface the fatal condition on the serial port via td-logger,
+    // independent of any surrounding logging that the caller may have done.
+    log::error!(
+        "MigTD fatal (code=0x{:x}): {} at {}:{}\n",
+        errorcode,
+        panic_message,
+        file,
+        line
+    );
+
     #[cfg(feature = "vmcall-raw")]
     {
         let crash_message = format!(