libxl: MigrateBegin: Dont call EndAPI in helper function

libxl: MigrateBegin: Dont call EndAPI in helper function

The libxlDomainMigrateBegin3Params API locks and ref counts the associated
virDomainObj but relies on the helper function libxlDomainMigrationBegin
to unref/unlock the object. libxlDomainMigrationBegin is also used by
libxlDomainMigratePerform3Params for p2p migration, but in that case the
lock/ref and unref/unlock are properly handled in the API entry point. So
p2p migrations suffer a double unref/unlock in the Perform API.

Remove the unref/unlock (virDomainObjEndAPI) from libxlDomainMigrationBegin
and adjust libxlDomainMigrateBegin3Params to properly unref/unlock
the virDomainObj on success and error paths.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>