xapi_vm_migrate: Fix reservations not being cleared on halted VMs

[last-genius]

Cross-pool migrations set scheduled_to_be_resident_on to the destination host, reserving memory and vGPUs. If the VM is still halted when migration is finished, the field is not cleared on the destination, preserving the reservations even though they're not necessarily going to be used anytime soon.

Call force_state_reset_keep_current_operations in pool_migrate_complete on the destination to clear the reservations among other things at the end of the migration.

This fixes an issue when VMs migrated across pools in a halted state would take up memory in xapi's view (but not in RRD's view), which is not intuitive and could prevent further migrations from claiming enough free memory on the host.

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I've tested this manually, confirming that migrating halted and running VMs works as designed, with the scheduled_to_be_resident_on field cleared at the end of the migration (in the first case it's because of the fix, in the second one it's because the VM is actually started on the host)

[robhoes]

I think this is safe. The function Xapi_vm_lifecycle.force_state_reset_keep_current_operations looks a little scary to call on running VMs, but it takes the power state into account. In fact, it is already called for running VMs in update_vm inside xapi_xenops.ml.