libxl: acquire job though begin phase only

libxl: acquire job though begin phase only

During migration, the libxl driver starts a modify job in the
begin phase, ending the job in the confirm phase.  This is
essentially VIR_MIGRATE_CHANGE_PROTECTION semantics, but the
driver does not support that flag.  Without CHANGE_PROTECTION
support, the job would never be terminated in error conditions
where migrate confirm phase is not executed.  Further attempts
to modify the domain would result in failure to acquire a job
after LIBXL_JOB_WAIT_TIME.

Similar to the qemu driver, end the job in the begin phase.
Protecting the domain object across all phases of migration can
be done in a future patch adding CHANGE_PROTECTION support.