CA-118762: Bring self.ty back

My previous pylint change 96e56bf7 replaced self.ty as it was not defined
anywhere in the VDI object. But this breaks some horrible hack in the
underlying layer, which was probably implementated that way for backward
"compatability" reason by itself.

Signed-off-by: Zheng Li <zheng.li@eu.citrix.com>
Reviewed-by: Vineeth Thampi Raveendran <vineeth.thampi@citrix.com>
Imported-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>

(cherry picked from commit 7c21ec7d2e2b5e00c099d489fb784fc024c585e7)