CA-100685: When domain disappears during VM shutdown operation, just go ahead instead of rasing any cancellation error

Import xen-api c/s 96cad346b8af1c98d253ee98e180fad4e5e98768

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Signed-off-by: Zheng Li <zheng.li@eu.citrix.com>