libelf: improve errors in elf_xen_note_check()

libelf: improve errors in elf_xen_note_check()

I recently debugged an isolated failure to boot, with no information other
than the logs.

The "Will only load images built for the generic loader or Linux images"
string was missing a newline, leading to the subsequent error being appended
to this line, rather than having its own line with correctly identified
function.

Furthermore, error messages which state "param containing $FOO is not $BAR" is
fairly useless for debugging without identifying which bad $FOO caused the
failure.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Keir Fraser <keir@xen.org>
CC: Jan Beulich <JBeulich@suse.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>