exec/rom_reset: Free rom data during inmigrate skip

Commit 355477f8 skips rom reset when we're an incoming migration
so as not to overwrite shared ram in the ignore-shared migration
optimisation.
However, it's got an unexpected side effect that because it skips
freeing the ROM data, when rom_reset gets called later on, after
migration (e.g. during a reboot), the ROM does get reset to the original
file contents.  Because of seabios/x86's weird reboot process
this confuses a reboot into hanging after a migration.

Fixes: 355477f8 ("migration: do not rom_reset() during incoming migration")
https://bugzilla.redhat.com/show_bug.cgi?id=1809380



Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/core/loader.c

@@ -1119,19 +1119,26 @@ static void rom_reset(void *unused)
 {
     Rom *rom;
 
-    /*
-     * We don't need to fill in the RAM with ROM data because we'll fill
-     * the data in during the next incoming migration in all cases.  Note
-     * that some of those RAMs can actually be modified by the guest on ARM
-     * so this is probably the only right thing to do here.
-     */
-    if (runstate_check(RUN_STATE_INMIGRATE))
-        return;
-
     QTAILQ_FOREACH(rom, &roms, next) {
         if (rom->fw_file) {
             continue;
         }
+        /*
+         * We don't need to fill in the RAM with ROM data because we'll fill
+         * the data in during the next incoming migration in all cases.  Note
+         * that some of those RAMs can actually be modified by the guest.
+         */
+        if (runstate_check(RUN_STATE_INMIGRATE)) {
+            if (rom->data && rom->isrom) {
+                /*
+                 * Free it so that a rom_reset after migration doesn't
+                 * overwrite a potentially modified 'rom'.
+                 */
+                rom_free_data(rom);
+            }
+            continue;
+        }
+
         if (rom->data == NULL) {
             continue;
         }