CA-103062: Fix for preventing vm pause during SXM

Issue: The vm is able to pause during SXM. This is happening because when we trigger an SXM, it first calls assert_can_migrate which is not a blocking operation. After it completes, then migrate_send, a blocking operation, is called and added to current_operations. When we trigger vm pause shortly after SXM has started, it is able to interfere the assert_can_migrate operation and does not migrate_send operate.
 
Fix : assert_can_migrate needs to be grouped with migrate_send operation inside with_vm_operation so that while SXM is in operation, vm pause should not interfere after assert_can_migrate completes or in between. I think fix is safe because it is not interfering with any other operation except SXM. Also the order of operations i.e asser_can_migrate followed by migrate_send remains same. With the fix , this order has become uninterruptible which I think is good. 

Signed-off-by: Ravi Pandey <ravi.pandey@citrix.com>