libxl: fix job handling across migration phases on dst

libxl: fix job handling across migration phases on dst

The libxlDomainMigrationDst* functions are a bit flawed in their
handling of modify jobs. A job begins when the destination host
begins receiving the incoming VM and ends after the VM is started.
The finish phase contains another BeginJob/EndJob sequence.

This patch changes the logic to begin a job for the incoming VM
in the prepare phase and end the job in the finish phase.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
ACKed-by: Michal Privoznik <mprivozn@redhat.com>