[SCSI] scsi_debug: Add "removable" parameter

Add "removable" module parameter to set the "removable" attribute of any
subsequently created debug block device. It is a writable driver option, so
that you can switch between removable and "fixed" media block devices in
between the add_host calls.

This is useful for being able to test the different behaviour/required
privileges in e. g. the udisks test suite.

Signed-off-by: Martin Pitt <martin.pitt@ubuntu.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>