kes Make DEVICE fd private. Its new name is m_fd, and can be

kes  Make DEVICE fd private. Its new name is m_fd, and can be
     obtained with dev->fd() outside the class.  This is the
     first of many DEVICE members to be made private.
kes  Implement Michael Renner's idea on having a virtual disk
     changer. I made some minor modifications (e.g. I did not
     implement the Virtual Changer directive, but simply set
     the Changer Command to an empty string.

git-svn-id: https://bacula.svn.sourceforge.net/svnroot/bacula/trunk@4203 91ce42f0-d328-0410-95d8-f526ca767f89