CA-189168: Close tapdisk in deactivate, not detach
CA-189168: Close tapdisk in deactivate, not detach

This ensures that, over a migrate, there's only one host with a tapdisk process
having a read/write filehandle open on the VHD.

This more closely mimics the SMAPIv2 behaviour.

jjd27/xapi-storage-datapath-plugins