xen/test/livepatch/Makefile: Install in DESTDIR/usr/lib/debug/xen-livepatch

xen/test/livepatch/Makefile: Install in DESTDIR/usr/lib/debug/xen-livepatch

Dumping these patch files in /usr/lib/debug/xen-*.livepatch is a bit
ugly.

Also, refactor the Makefile to have a LIVEPATCHES variable, to reduce
repetition.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>
(cherry picked from commit a38d1af5fb02bee68c9a30e38b97c6129815f943)