The virErrorDomain enum has VIR_FROM_XEN, VIR_FROM_XEND,
VIR_FROM_XENSTORE, VIR_FROM_SEXPR, and VIR_FROM_XENXM. Use
these elements in the corresponding .c files. While at it,
remove the VIR_FROM_THIS define in src/xenconfig/xenxs_private.h.