CP-6271: Set default mode for XSM checks to use UUIDs

After this change the code at xapi_vm_migrate.ml:362 will
check if the VDI exists (by UUID) in the destination SR.
If it does, a mirror will not be performed as the VDI already exists.

The original behaviour can be used by setting the value to false in xapi.conf

Signed-off-by: Bob Ball <bob.ball@citrix.com>
Merge pull request #1498 from BobBall/cp-6271

CP-6271: Set default mode for XSM checks to use UUIDs