After Powering up the servers even couples of times. You might have a strange boot up issue with your Hypervisor and and IBM XSeries with an EFI Firmware.
During the last stage of the Boot Sequence you might received :
Hypervisor : Boot Failed or the system loop and restart.
To avoid this behavior, Go to your System Boot Sequence and ADD : Legacy Only as the first Boot Device.
This will force the EFI to turn off and boot as a Normal “Legacy” Bios and Power up again your Hypervisor !
Thanks to the IBM Technical support for this one.
For more information read page 5 (or the whole document): introducing_uefi-compliant_firmware_on_ibm_system_x.1.0