Part of a series of cleanups to use new accessor methods.

* src/xenxs/xen_sxpr.c (xenParseSxprDisks, xenParseSxpr)
(xenFormatSxprDisk, xenFormatSxpr): Use accessors.
* src/xenxs/xen_xm.c (xenParseXM, xenFormatXMDisk, xenFormatXM):
Likewise.
Signed-off-by: NEric Blake <eblake@redhat.com>