xen: Install unstripped version -syms version into /usr/lib/debug

xen: Install unstripped version -syms version into /usr/lib/debug

xen-*-syms cannot actually be booted, so putting it in /boot is a bit
misleading. It also happens to cause a warning from update-grub (so at
least it doesn't end up in grub.cfg)

/usr/lib/debug seems to be a pretty common path for installing such
debug info.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
[ ijc -- fixed typos ]