Config drive attached as cdrom

Some guest OSes (e.g. Windows) require config drive to be a cdrom device
to access the iso9660 filesystem on it. Option config_drive_format is
therefore set to also control the device type of config drive. If it's
set to 'iso9660', config drive will be cdrom; if it's 'vfat', config
drive will be disk.

DocImpact
Fixes: bug #1155842
Change-Id: I6c08b1b8040a1fd0db8e2b3b1fc798060733001f
Merge "Config drive attached as cdrom"