[IA64] Move nvram from /usr to /var

[IA64] Move nvram from /usr to /var

Presently nvram is stored in /usr/lib/xen/boot/nvram_<domain> next to the guest
firmware.  This violates the FHS because /usr might be mounted read-only.  This
patch moves the nvram storage to /var/lib/xen/nvram/nvram_<domain>

Also clean up:
- references to stat_buf assumed that stat() had succeeded; use access()
  instead since it's easier and doesn't require stat_buf at all
- nvram_path[PATH_MAX] instead of nvram_path[100]
- strncpy(..., strlen(src)) is meaningless, re-order length tests to work
  correctly

Signed-off-by: Aron Griffis <aron@hp.com>