Fix QSPI::isNull()

The method QSPI::isNull() should return false when QSPI is empty.

Tested on Winsows 8 and Linux with on-board, virtual and USB serial
ports using Qt5 and then Qt4.

Task-number: QTBUG-41262
Change-Id: Ic9e0afc5549311283aef7ec2ed841f5e418b7edf
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Create notifiers inside of startAsyncXX() methods when required

It is reasonable to move creation of notifiers from the constructor
to the specific separate methods. In this case notifiers are created
in startAsyncXXX() methods if necessary for concrete I/O operation.
Also it allows to return an error in case a notifier is failed.

Tested on Windows 8 with virtual com0com serial ports.

Change-Id: I351a4c54214c84455150d0fc157a99108280cc12
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Declare QElapsedTimer closer to usage

Change-Id: I74622eaf3a7f5179ff25fc815997d5e4cf3cc2c7
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Simplify handling of overlapped result

There is not necessary to pass direction to handleOverlappedResult().
Because it is passed already as a second argument (pointer to
OVERLAPPED structure that defines a desired direction).

Also this method is renamed to more simple and short name.

Change-Id: I9c20cf97c1712aed8d3e9ea6d9b4687ce4487523
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Remove Q_OS_WIN64 since Q_OS_WIN32 defines 64-bit too

Change-Id: I32eaf132f19a3b6fcbd5fd55d0690c3bd62bb641
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>