xentrace: install into sbin

xentrace: install into sbin

Collecting the trace buffer requires root permissions. Adjust Makefile
to install xentrace and xentrace_setsize into sbindir. Leave the
existing support for BIN in place for upcoming changes.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: George Dunlap <george.dunlap@eu.citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>