qga-win: Fix guest-get-devices error API violations

The Error ** argument must be NULL, &error_abort, &error_fatal, or a
pointer to a variable containing NULL.  Passing an argument of the
latter kind twice without clearing it in between is wrong: if the
first call sets an error, it no longer points to NULL for the second
call.

qmp_guest_get_devices() is wrong that way: it calls error_setg() in a
loop.

If no iteration fails, the function returns a value and sets no error.
Okay.

If exactly one iteration fails, the function returns a value and sets
an error.  Wrong.

If multiple iterations fail, the function trips error_setv()'s
assertion.

Fix it to return immediately on error.

Perhaps the failure to convert the driver version to UTF-8 should not
be an error.  We could simply not report the botched version string
instead.

Drop a superfluous continue while there.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>

qga/commands-win32.c

@@ -2385,7 +2385,7 @@ GuestDeviceInfoList *qmp_guest_get_devices(Error **errp)
         device->driver_name = g_utf16_to_utf8(name, -1, NULL, NULL, NULL);
         if (device->driver_name == NULL) {
             error_setg(errp, "conversion to utf8 failed (driver name)");
-            continue;
+            return NULL;
         }
         slog("querying device: %s", device->driver_name);
         hw_ids = ga_get_hardware_ids(dev_info_data.DevInst);
@@ -2428,7 +2428,7 @@ GuestDeviceInfoList *qmp_guest_get_devices(Error **errp)
             NULL, NULL);
         if (device->driver_version == NULL) {
             error_setg(errp, "conversion to utf8 failed (driver version)");
-            continue;
+            return NULL;
         }
         device->has_driver_version = true;
 
@@ -2452,7 +2452,6 @@ GuestDeviceInfoList *qmp_guest_get_devices(Error **errp)
             cur_item->next = item;
             cur_item = item;
         }
-        continue;
     }
 
     if (dev_info != INVALID_HANDLE_VALUE) {