xen_xm: Split the per-disk logic from xenParseXMDisk()

xen_xm: Split the per-disk logic from xenParseXMDisk()

xenParseXMDisk() does a lot of stuff and, in order to make things
cleaner, let's split it in two new functions:
- xenParseXMDisk(): it's a new function that keeps the old name. It's
responsible for the whole per-disk logic from the old xenParseXMDisk();
- xenParseXMDiskList(): it's basically the old xenParseXMDisk(), but
now it just iterates over the list of disks, calling xenParseXMDisk()
per each disk.

This patch is basically preparing the ground for the future when
typesafe virConf acessors will be used.

Signed-off-by: Fabiano Fidêncio <fabiano@fidencio.org>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>