QNonContiguousByteDevice: reset() instead of seek() if possible

... because e.g. QHttpMultiPartIODevice does not implement seek at all
(QTBUG-30295), which would make resetting such a device fail.

Change-Id: I0a9c0f21047baa72146690bfdf638cdffab0e25f
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>
Reviewed-by: Shane Kearns <shane.kearns@accenture.com>

src/corelib/io/qnoncontiguousbytedevice.cpp

@@ -388,8 +388,8 @@ bool QNonContiguousByteDeviceIoDeviceImpl::reset()
 {
     if (resetDisabled)
         return false;
-
-    if (device->seek(initialPosition)) {
+    bool reset = (initialPosition == 0) ? device->reset() : device->seek(initialPosition);
+    if (reset) {
         eof = false; // assume eof is false, it will be true after a read has been attempted
         return true;
     }