[CA-106120] & [CA-107155]: Remove stale references of the host to the VDI in VDI.attach.

This is a combination of two commits from trunk, 436156902d9f and
4f89c29f1917.
The latter is required to fix a regression introduced by the former.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Andrei Lifchits <andrei.lifchits@citrix.com>
Reviewed-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Imported-by: Thanos Makatos <thanos.makatos@citrix.com>

GitHub: closes #123 on xapi-project/sm