libxl: fix logic in P2P migration

libxl: fix logic in P2P migration

libxlDoMigrateSrcP2P() performs all phases of the migration
protocol for peer-to-peer migration. Unfortunately the logic
was a bit flawed since it is possible to skip the confirm
phase after a successfull begin and prepare phase. Fix the
logic to always call the confirm phase after a successful begin
and perform. Skip the confirm phase if begin or perform fail.

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