s390/cio: export vpm via sysfs

Add new attribute "vpm" to the subchannel sysfs directory of I/O
subchannels. This attribute contains a path mask indicating which
channel paths were successfully verified to be usable for I/O.

Reviewed-by: Peter Oberparleiter <peter.oberparleiter@de.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>