Add the serial port to the VM using the steps below (Nutanix does not automatically add serial ports in AOS version 4.6 and later. Omitting this step may result in NetScaler VM boot error "Loading /boot/defaults/loader.conf"):

below Steps needs to be followed 

1. Turn off the VM.
2. SSH to one of the CVMs (Controller VMs) in the AHV cluster.
3. Run the following command to add the serial port to the VM:

   acli vm.serial_port_create <vm_name> type=kServer index=0

   Replace <vm_name> with the name of the VM.
   You must create a new VM to pass the serial_port_create properly. Updating an existing VM disk might not work in some cases.

Upgrade or reboot scenario

When you reboot the new VM or upgrade the Nutanix hosted NetScaler VPX appliance, the serial port is lost, causing the same situation Error: "Loading /boot/defaults/loader.conf". 

SeaBIOS (version 1.9.1-5.e16)
Machine UUID f359aaca-aaaa-4a62-bc3c-310cf1c4da22

iPXE (http://ipxe.org) 00:03.0 C980 PC12.10 PnP PMM+BFF9F3E0+BFEFF3E0 (980

Booting from Hard Disk...
BTX loader 1.00 BTX version is 1.02
Consoles: internal video/keyboard 
BIOS drive C: is diske
BIOS 639kB/3144572kB available memory

FreeBSD/x86 bootstrap loader, Revision NS1.2 
Loading /boot/defaults/loader.conf