(FACT-654) Don't try to resolve Ldom facts on non-SPARC systems

In Solaris 11.2, Oracle introduced a rewrite of virtinfo which
supports both x86 and SPARC, whereas the old version only supported
SPARC. Since Ldoms are a SPARC-only feature, we now need to confine
the ldom facts to only resolve on SPARC based platforms.

Note that this information comes from community member Kristina Tripp,
who provided the necessary details on the new virtinfo implementation.