target-ppc: Move cpu_exec_init() call to realize function

Move cpu_exec_init() call from instance_init to realize. This allows
any failures from cpu_exec_init() to be handled appropriately.
Also add corresponding cpu_exec_exit() call from unrealize.

cpu_dt_id assignment from instance_init is no longer needed since
correct assignment for cpu_dt_id is already present in realizefn.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
[AF: Keep calling cpu_exec_init() for CONFIG_USER_ONLY]
Signed-off-by: Andreas Färber <afaerber@suse.de>
