Detect a valid of serial8250 ports

Usually, a Linux kernel reserves a places for the non-PnP serial
devices, for example, such as ttyS[0-4/14]. In case the system has not a
HW devices which are attached to this places, then these devices will be
stubs and present as usual serial ports. Thus, we can know about a
validity of these devices only in attempt to open them and to query a
type of.

Note: This patch is theoretical, because are not tested for a valid 8250
devices.

Task-number: QTBUG-32024
Change-Id: I313145caadfeeee4d5fe3c8e16c1bcf4ae79214d
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>