Fix #605 by closing current device when another is selected

As the device manager class holds a pointer to all devices
they will never be destroyed until PV exits. Devices are
opened with the first call to create() but only closed in
the destructor. Together, this results in devices never
being closed.

This patch fixes this by renaming create() to open()
and introducing a matching close() method that the
session class calls when a different device is to be
selected.