Show a % when a volume is loaded in a drive.

For a status slots print a % when the volume is loaded into
a drive.

So now we have:

'@' - import export slot
'*' - data in backup catalog doesn't match
'%' - volume is loaded into a drive.
' ' - nothing special with volume just in slot and catalog info matches.

Signed-off-by: Marco van Wieringen <marco.van.wieringen@bareos.com>