[LIBELF] Prefer PT_NOTE segments to SHT_NOTE sections for ELF notes.

[LIBELF] Prefer PT_NOTE segments to SHT_NOTE sections for ELF notes.

It's always an error to try to use sections on an executable; the
segments in the phdr are definitive.

Unfortunately we cannot drop SHT_NOTE support completely due to a
binutils bug which causes kernels to have the offset field of the
PT_NOTE phdr set to zero:

	http://sourceware.org/bugzilla/show_bug.cgi?id=594

This bug is present in binutils 2.17 although some distros have
backported the fix.

Therefore we simply prefer a PT_NOTE segment if we find one otherwise
we still use the SHT_NOTE section (and then the old __xen_guest
section).

Based on a patch from Jeremy Fitzhardinge.

Also added XEN_ELFNOTE_HV_START_LOW to readnotes.

Signed-off-by: Ian Campbell <ian.campbell@xensource.com>