CONFIG_SUN_LDOMS is not available for thedefaultarchitecture x86.
Result is shown for architecture 64BIT or sparc

Sun Logical Domains support

modulename: ldc.ko

configname: CONFIG_SUN_LDOMS

Linux/SPARC Kernel Configuration
└─>Bus options (PCI etc.)
└─>Sun Logical Domains support
In linux kernel since version 2.6.23 (release Date: 2007-10-09)  
Say Y here is you want to support virtual devices via
Logical Domains.

source code:
arch/sparc/kernel/
depends
CONFIG_SPARC64